Let users auto follow specific members

Hello,

I want members after their registration to auto follow specific users. I found this code on the web but without any instructions where I should implement it.

<?php
add_action('um_after_new_user_register','um_auto_follow_artists',10,2);
function um_auto_follow_artists( $user_id, $args ){
    global $ultimatemember, $um_followers;

	 $follow_users = array( 
		'rjtmusic',
		'james-peacock',
		'carl-thomson',
		'jonathan-proud',
		'stu-dawson',
		'carly-mckee',
	 );

	 foreach( $follow_users as $username ){
	    $follow_user = get_user_by( 'login', $username );
	 	$um_followers->api->add( $follow_user->ID, $user_id );
	 }

 }

?>