add_query_arg not work in admin

some_body needs to be run as an action on pre_get_posts.

Here’s an example of the entire process of adding new query vars.

Pay extra attention to myplugin_pre_get_posts:

function myplugin_pre_get_posts( $query ) {
...
    $city = get_query_var( 'city' );
...
}
add_action( 'pre_get_posts', 'myplugin_pre_get_posts', 1 );