SubSubForm in LibreBase

I working with libre Base 4.2. I have a MainForm. There can a user select from a list a domain. On the MainForm is a Subform. In the subform the user can select a Usergorup. Now I create a Subform in the first subform (User can select a username - many to may relation table). But I cannot connect the subform with the subsubform.

I using ODBC to connect with a MS SQL-Server 2012

All Table Relations and FK a set on the MSSQL-side and shown in libreBase under the Relations-View

Where can I find a working example for my szenario?

Freundliche Grüße / Best regards

In the past I've always found this example useful for sub forms

https://forum.openoffice.org/en/forum/viewtopic.php?f=100&t=59195

Regards