//Add Role To Register Link
add_action('register_form', 'register_role_custom');

function register_role_custom(){ ?>

    <input id="role" type="hidden" tabindex="0" size="0" value= "<?php if (isset($_GET['role'])){echo $_GET['role'];} ?>" name="role"/>

<?php
}

add_action('user_register', 'custom_user_role');

function custom_user_role($user_id, $password="", $meta=array()) {

    $userdata = array();
    $userdata['ID'] = $user_id;
    $userdata['role'] = $_POST['role'];

    if (($userdata['role'] == "employer") or ($userdata['role'] == "candidate"))  {
    wp_update_user($userdata);
    }
}

//2. Add validation. In this case, we make sure first_name is required.
add_filter('registration_errors', 'custom_registration_errors', 10, 3);
function custom_registration_errors ($errors, $sanitized_user_login, $user_email) {

    if (empty( $_POST['role'])){
    $errors->add( 'no_role_error', __('<strong>ERROR</strong>: No Role Set in URL','mydomain') );
    return $errors;
    }

}