Redirect to External URL

Hi. I’m trying to write a simple plugin to redirect a user to their home site on a multisite if they log in on a multisite. So basically if they go to mymultisite.com and login, I want them to be redirected to the multisite they registered to (their source domain) so like mysite.mymultisite.com.

This is what I have:

<?php

add_filter( 'login_redirect', 'send_subscribers_home', 10, 3 );

function send_subscribers_home( $location, $request, $user ) {
    global $user;
    if ( isset( $user->roles ) && is_array( $user->roles ) ) {
        if ( in_array( 'customer', $user->roles ) ) {
			$user_ID = get_current_user_id();
			$homeRedirect = get_user_meta($user_ID, 'source_domain');
			$homeRedirectFinal = json_encode($homeRedirect);
            return $homeRedirectFinal;
        } else {
            return $redirect_to;
            }
    }
    return;
}

?>

So basically I’m intercepting the login_redirect filter, checking if the user is in a certain group, and if so, grabbing the website they originally registered to (in this case it will be a subdomain) and redirecting that.

I’ve never written a WordPress plugin before. Anyways the problem is that this returns “www.mymultisite.com/www.mysite.mymultisite.com” as a web page, and obviously throws a 404. Does anyone know if I can change how WordPress parses this address (so it’s not as a page of the main site, but rather just an external URL) without modifying wp-login.php ?

Thanks.