Hook um_main_{$mode}_fields

um_main_mode_fields.php

<?php

/**
 * Hook: um_main_{$mode}_fields
 *
 * Type: action
 *
 * Description: Fires before rendering main form fields.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/templates/login.php#L71
 * @link    https://docs.ultimatemember.com/article/1171-ummainmodefields
 *
 * @package um\templates
 * @see     templates/login.php
 * @see     templates/register.php
 * @see     um_profile_content_main()
 * @since   2.0
 * @version 3.0
 *
 * @param array $args Form shortcode arguments.
 */
function my_main_login_fields( $args ) {
	// your code here.
}
$mode = 'login';
add_action( "um_main_{$mode}_fields", 'my_main_login_fields', 10, 1 );