Hook um_before_{$mode}_form_is_loaded

um_before_mode_form_is_loaded.php

<?php

/**
 * Hook: um_before_{$mode}_form_is_loaded
 *
 * Type: action
 *
 * Description: Fires before the form shortcode is loaded.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-shortcodes.php#L792
 * @link    https://docs.ultimatemember.com/article/1072-umbeforemodeformisloaded
 *
 * @package um\core
 * @see     um\core\Shortcodes::load()
 * @since   2.0
 * @version 3.0
 *
 * @param array $args Form shortcode arguments.
 */
function my_before_login_form_is_loaded( $args ) {
	// your code here.
}
$mode = 'login';
add_action( "um_before_{$mode}_form_is_loaded", 'my_before_login_form_is_loaded', 10, 1 );