Hook um_custom_dropdown_options_source_url__{$form_key}

um_custom_dropdown_options_source_url__form_key.php

<?php

/**
 * Hook: um_custom_dropdown_options_source_url__{$form_key}
 *
 * Type: filter
 *
 * Description: Change custom dropdown options source URL by $form_key.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-fields.php#L3139
 * @link    https://docs.ultimatemember.com/article/1094-umcustomdropdownoptionssourceurlformkey
 *
 * @package um\core
 * @see     um\core\Fields::edit_field()
 * @since   2.0
 * @deprecated
 *
 * @param string $url  Dropdown options source URL.
 * @param array  $data Field data.
 *
 * @return string Dropdown options source URL.
 */
function my_custom_dropdown_options_source_url( $url, $data ) {
	// your code here.
	return $url;
}
$form_key = 'role';
add_filter( "um_custom_dropdown_options_source_url__{$form_key}", 'my_custom_dropdown_options_source_url', 10, 1 );