Fortigate Terraform Provider - FSSO Groups to Policies


I have a Fortigate FortiWiFi 60E with Firmware v6.2.3.

I am using:
Terraform v0.13.5

I sucessfully connected the fortios provider. I can now configure policies with terraform.

What I cannot find is a way to configure a FSSO Group to a policy source. Within the Fortigate GUI I can easily add it to a Source Object. (The connector is configured and alive)

Has somebody done this before and has eventueally a hint to the correct doc or better an example to share?

Thanks a lot!