Prepopulating (webform) checkboxes

This may apply to all checkboxes but I couldn't work out how to prepopulate specifically webform checkboxes. If I change the end of the module to:

else {
if ($form['#type'] == 'checkboxes') {
$form['#default_value'][] = $getslice;
} else {
$form['#default_value'] = $getslice;
}
}

Then I can do something like:
contact-us?edit[submitted][mycheckboxes]=checkbox