function apply_my_optin_tag( $update_data, $user_id, $contact_id, $form_id ) {

    if ( $update_data['gh_field_name'] == 'yes' ) {

        $tag_id = wp_fusion()->user->get_tag_id( 'Name of tag' );

        wp_fusion()->crm->apply_tags( array( $tag_id ), $contact_id );

    }

}

add_action( 'wpf_forms_post_submission', 'apply_my_optin_tag', 10, 4 );