How to use options from an external webservice

To populate webform options from an external webservice you need to create an empty set of webform options by going to /admin/structure/webform/config/options/manage/add 
and then populating these options using hook_webform_options_WEBFORM_OPTIONS_ID_alter().

Here is an example of an empty webform optiond config call 'External Countries' (machine_name: external_countries). The complete config name is webform.webform_options.external_countries.yml

langcode: en
status: true
dependencies: {  }
id: external_countries
label: 'External Countries'
category: ''
likert: false
options: ''

Here is a very simple example of using hook_webform_options_WEBFORM_OPTIONS_ID_alter() to populates the External Countries (external_countries) webform options using an external list of U.S. States in JSON.

/**
 * Implements hook_webform_options_WEBFORM_OPTIONS_ID_alter().
 */
function CUSTOM_MODULE_webform_options_external_countries_alter(array &$options, array &$element) {
  // Load cached options.
  if ($cache = \Drupal::cache()->get('external_countries')) {
    $options = $cache->data;
    return;
  }

  // Get data as associative array from the external webservice.
  $external_countries_url = 'https://gist.githubusercontent.com/mshafrir/2646763/raw/8b0dbb93521f5d6889502305335104218454c2bf/states_hash.json';
  $options = json_decode(file_get_contents($external_countries_url), TRUE);

  // Cache options so that we don't have to make continual requests
  // to the external webservice.
  \Drupal::cache()->set('external_countries', $options);
}