Select Only one category on Form

Hi,
you can do that either with the Custom Fields extension or by adding the code below in your theme functions.php file (or even better by creating a new blank plugin and pasting the code there https://wpadverts.com/blog/how-to-use-code-snippets-in-wordpress/).

add_filter( "adverts_form_load", "single_select_advert_category" );
function single_select_advert_category( $form ) {
  if( $form['name'] != "advert" ) {
    return $form;
  }
  foreach( $form["field"] as $key => $field ) {
    if( $field["name"] == "advert_category" ) {
        $form["field"][$key]["empty_option"] = true;
        $form["field"][$key]["max_choices"] = 1;
        $form["field"][$key]["validator"] = array();
        $form["field"][$key]["validator"][] = array( 
            "name" => "max_choices",
            "params" => array( "max_choices" => 1 )
        );
    }
  }
  return $form;
}

Thank you so much.