E-mail signature in Interactive Service Hub

Hello everybody,

I want to put a default e-mail signature in a e-mail when i create it. In the normal web client it works well: A default signature appears in every new e-mail. There is also a button at the top of my screen, where i can choose a different signature. 

This does not work in the Interactive Service Hub. Does anybody have an idea how to fix this?

Thanks a lot!