How to assign a user role on frontend registration only

Hello,
I appreciate your question, well

Here’s an example of how you can achieve this by using the is_admin() function:

function custom_user_register($user_id) {
   
    if (is_admin()) {
       echo "hello";
    } else {
        $role_to_assign = 'your_frontend_role';
        wp_update_user(array('ID' => $user_id, 'role' => $role_to_assign));
    }
}

add_action('user_register', 'custom_user_register');

Adjust the code according to your specific needs and the roles you want to assign. you can set value instead of your_frontend_role in above code like admin, editor any role you want, just add there.
Hope you find your answer. Thank you

Using is_admin() was my initial thought as well. But it might not be effective, depending on how front end forms manage registration. It’s possible for a front end form to register by making an admin request, in which case checking is_admin() would be ineffective.

It might be necessary to check the referrer page; or alter the front end form to include a hidden field so your registration callback can easily distinguish front end registration requests.