Cant select dynamic "Interface adress" in Outbound NAT Rule after setting up Interface Group

[Circus0640]

Happy new year netgate community!

My scenario is the following:
I want to use several mullvad wireguard tunnels.

I set up the Interfaces, Gateways etc. all without a single problem.
Handshake is fine.

I am running into problems trying to set the Outbound NAT Rule.
I am able to select the Interface Group just fine, but I cant select something dynamic as Translation adress. I am only able to pick the static Adresses of the seperate Interfaces.

I am following this guide link text and you can see he set the Interface adress as dynamic at 24:56.

What am I missing here?

Thank you for all your help in advance!
Have a nice day!