Hello, folks! Could you please provide guidance on how to perform an Insert or Update for a custom field?

Do you need this to add an ad? It's easy to implement this function. Specify your fields in the endpoint, and specify your field type in the classes when processing. The call is like this: &object=item&action=metaById&itemId=
I managed to display ads on the application screen and, after a bit of trouble, I displayed the input fields on the ad adding screen.