Add new record with specific form

Hi,

We are facing a weird problem ussing forms in a custom entity in CRM 2016 (8.2.0.749) on premise.

We need to fill information in a custom entity using a specific form, different from the "standard" one.

Then we need to create a second record, which is related to the first one and must be created using the same form.

But when we press "add new record" from the record 1 using specific form, the record 2 is opened in a new windows with the standard form.

Can you give me some info about this?

Thank you :)