Master / Details with using one to one single record

I need to use a one to one form with the rows side limited to a singular record form. It seems strange but I need to have this limit of one record only on rows.
On recent version of SC I read it’s possible but now I’m not able to insert the row side of form.
I manage all correctly but SC do not save the row record on database and do not show the insert record also if it’s set to on.
Am I missing something ??
Is there a sample to see ?