Use at your own discretion

The code requires a Select Field with Title Select a User and one option set.
It then completes the options of the Select Field with User Login Names as labels and User IDs as values.

Cannot be used for Views Custom Searches.

add_filter( 'wpt_field_options', function ( $current_options, $title_of_field ) {
    if ( $title_of_field != 'Select a User' ) {
        // not our "Headline Color" field
        return $current_options;
    }

    $users_objects_array = get_users();
    $users = array();

    foreach ($users_objects_array as $user_object_array) {
    	$users[$user_object_array->data->user_login] = $user_object_array->ID;
    }
    //error_log(print_r($user_id_array, true));

    if ( ! $users_objects_array ) {
        // no theme colors are set yet
        return array(
            array(
                '#title' => 'No users available',
                '#value' => 0
            )
        );
    }

    $new_options = array();
    foreach ( $users as $user => $user_value ) {
        $new_options[] = array(
            '#title' => $user . '-' . $user_value,
            '#value' => $user_value
        );
    }
    

    return $new_options;
}, 30, 2 );