function my_capabilities_field( $user_meta, $user_id ) {

    if ( ! empty( $user_meta['wp_ls_capabilities'] ) ) {
        $user_meta['wp_ls_capabilities'] = array_keys( $user_meta['wp_ls_capabilities'] );
    }

    return $user_meta;

}

add_filter( 'wpf_user_update', 'my_capabilities_field', 10, 2 );
add_filter( 'wpf_user_register', 'my_capabilities_field', 10, 2 );