custom field only for backend

Hi,
it is the easiest to do that with the Custom Fields extension https://wpadverts.com/extensions/custom-fields/ when creating a new field it will allow you to select where the field should be show, one of the options is the “Admin form only”.

If you are working with the Forms API https://wpadverts.com/doc/custom-fields/ to add a new field then inside of your adverts_form_load filter you can use the is_admin() function to check if the field should be added to the form

add_filter( "adverts_form_load", function( $form ) {
  if( ! is_admin() ) {
    return $form;
  }
  // add here the field only admin should see
  return $form;
} );