You can use a hook to determine if the user is login from the login page or from widget of any other page and then redirect as per your choice.
function my_login_redirect( $redirect_to, $request, $post ) {
//is there a page to check
global $post;
if($post->ID == page id of your login/register page) {
return get_bloginfo(‘url’) . “/custompage/”;
} else {
return $redirect_to;
}
}
add_filter( ‘login_redirect’, ‘my_login_redirect’, 10, 3 );
I haven’t tested the code …. but you can copy paste this code in your theme’s function.php and let me know. Remember to replace the id and custom page with your page ID and custom page slug name.