diff --git a/core/modules/media_library/src/Form/AddFormBase.php b/core/modules/media_library/src/Form/AddFormBase.php
index 4e175a9a39..5fa5038659 100644
--- a/core/modules/media_library/src/Form/AddFormBase.php
+++ b/core/modules/media_library/src/Form/AddFormBase.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
 use Drupal\media\MediaInterface;
 use Drupal\media\MediaTypeInterface;
 use Drupal\media_library\Ajax\UpdateSelectionCommand;
@@ -125,6 +126,13 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     $form['#suffix'] = '</div>';
     $form['#attached']['library'][] = 'media_library/style';
 
+    // Since the media library can be loaded via AJAX, we force the action URL
+    // of the add form to the media library controller so we now for sure the
+    // form is actually on the page.
+    $form['#action'] = Url::fromRoute('media_library.ui', [], [
+      'query' => $form_state->get('media_library_state')->all(),
+    ])->toString();
+
     // The form is posted via AJAX. When there are messages set during the
     // validation or submission of the form, the messages need to be shown to
     // the user.
@@ -585,7 +593,7 @@ public function updateWidget(array &$form, FormStateInterface $form_state) {
     }
 
     // Pass the selection to the field widget based on the current widget ID.
-    $opener_id = MediaLibraryState::fromRequest($this->getRequest())->getOpenerId();
+    $opener_id = $form_state->get('media_library_state')->getOpenerId();
     if ($field_id = MediaLibraryWidget::getOpenerFieldId($opener_id)) {
       // The added media items get an ID  when they are saved in ::submitForm().
       // For that reason the added media items are keyed by delta in the form
diff --git a/core/modules/media_library/src/Form/FileUploadForm.php b/core/modules/media_library/src/Form/FileUploadForm.php
index 2a306b9ebe..f28376e237 100644
--- a/core/modules/media_library/src/Form/FileUploadForm.php
+++ b/core/modules/media_library/src/Form/FileUploadForm.php
@@ -8,9 +8,11 @@
 use Drupal\Core\Field\TypedData\FieldItemDataDefinition;
 use Drupal\Core\File\Exception\FileWriteException;
 use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\ElementInfoManagerInterface;
 use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Url;
 use Drupal\file\FileInterface;
 use Drupal\file\Plugin\Field\FieldType\FileFieldItemList;
 use Drupal\file\Plugin\Field\FieldType\FileItem;
@@ -200,6 +202,16 @@ public function processUploadElement(array $element, FormStateInterface $form_st
     $element['upload_button']['#ajax'] = [
       'callback' => '::updateFormCallback',
       'wrapper' => 'media-library-wrapper',
+      // Add a fixed URL to post the form since AJAX forms are automatically
+      // posted to <current> instead of $form['#action'].
+      // @todo Remove when https://www.drupal.org/project/drupal/issues/2504115
+      //   is fixed.
+      'url' => Url::fromRoute('media_library.ui'),
+      'options' => [
+        'query' => $form_state->get('media_library_state')->all() + [
+          FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
+        ],
+      ],
     ];
     return $element;
   }
diff --git a/core/modules/media_library/src/Form/OEmbedForm.php b/core/modules/media_library/src/Form/OEmbedForm.php
index 0361c1f0e9..ffdc6950c9 100644
--- a/core/modules/media_library/src/Form/OEmbedForm.php
+++ b/core/modules/media_library/src/Form/OEmbedForm.php
@@ -3,7 +3,9 @@
 namespace Drupal\media_library\Form;
 
 use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
 use Drupal\media\OEmbed\ResourceException;
 use Drupal\media\OEmbed\ResourceFetcherInterface;
 use Drupal\media\OEmbed\UrlResolverInterface;
@@ -121,6 +123,16 @@ protected function buildInputElement(array $form, FormStateInterface $form_state
       '#ajax' => [
         'callback' => '::updateFormCallback',
         'wrapper' => 'media-library-wrapper',
+        // Add a fixed URL to post the form since AJAX forms are automatically
+        // posted to <current> instead of $form['#action'].
+        // @todo Remove when https://www.drupal.org/project/drupal/issues/2504115
+        //   is fixed.
+        'url' => Url::fromRoute('media_library.ui'),
+        'options' => [
+          'query' => $form_state->get('media_library_state')->all() + [
+            FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
+          ],
+        ],
       ],
       '#attributes' => [
         'class' => ['media-library-add-form-oembed-submit'],
diff --git a/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
index e2292e3b73..043c5daf0c 100644
--- a/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
+++ b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
@@ -723,7 +723,7 @@ public function testWidgetUpload() {
     $assert_session->pageTextNotContains($png_image->filename);
 
     // Assert we can also directly insert uploaded files in the widget.
-    $assert_session->elementExists('css', '.media-library-open-button[href*="field_twin_media"]')->click();
+    $assert_session->elementExists('css', '.media-library-open-button[name^="field_twin_media"]')->click();
     $assert_session->assertWaitOnAjaxRequest();
     $assert_session->pageTextContains('Add or select media');
     $page->clickLink('Type Three');
@@ -839,7 +839,7 @@ public function testWidgetUpload() {
 
     // Assert we can also remove selected items from the selection area in the
     // upload form.
-    $assert_session->elementExists('css', '.media-library-open-button[href*="field_unlimited_media"]')->click();
+    $assert_session->elementExists('css', '.media-library-open-button[name^="field_unlimited_media"]')->click();
     $assert_session->assertWaitOnAjaxRequest();
     $assert_session->pageTextContains('Add or select media');
     $page->clickLink('Type Three');
@@ -1007,7 +1007,7 @@ public function testWidgetOEmbed() {
     $assert_session->pageTextContains('Custom video title');
 
     // Assert we can directly insert added oEmbed media in the widget.
-    $assert_session->elementExists('css', '.media-library-open-button[href*="field_unlimited_media"]')->click();
+    $assert_session->elementExists('css', '.media-library-open-button[name^="field_unlimited_media"]')->click();
     $assert_session->assertWaitOnAjaxRequest();
     $assert_session->pageTextContains('Add or select media');
     $page->clickLink('Type Five');
@@ -1022,7 +1022,7 @@ public function testWidgetOEmbed() {
 
     // Assert we can remove selected items from the selection area in the oEmbed
     // form.
-    $assert_session->elementExists('css', '.media-library-open-button[href*="field_unlimited_media"]')->click();
+    $assert_session->elementExists('css', '.media-library-open-button[name^="field_unlimited_media"]')->click();
     $assert_session->assertWaitOnAjaxRequest();
     $assert_session->pageTextContains('Add or select media');
     $page->clickLink('Type Five');