Hook um_custom_dropdown_options_source__{$form_key}

um_custom_dropdown_options_source__form_key.php

<?php

/**
 * Hook: um_custom_dropdown_options_source__{$form_key}
 *
 * Type: filter
 *
 * Description: Change custom dropdown options source by $form_key.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-fields.php#L3114
 * @link    https://docs.ultimatemember.com/article/1093-umcustomdropdownoptionssourceformkey
 *
 * @package um\core
 * @see     um\core\Fields::edit_field()
 * @since   2.0
 * @version 3.0
 *
 * @param string $source Dropdown options source.
 * @param array  $data   Field data.
 *
 * @return string Dropdown options source.
 */
function my_custom_dropdown_options_source( $source, $data ) {
	// your code here.
	return $source;
}
$form_key = 'role';
add_filter( "um_custom_dropdown_options_source__{$form_key}", 'my_custom_dropdown_options_source', 10, 2 );