Hi,
I also searched this option "for ages...", but I found it ;-)
You have to log on (SOGo WebGUI) with the user that is wanted as the sender
In the "Mail-View" please click on the "points" next to the Mailaddress and select "Delegation".
There you have to enter the delegate.
Afterwards set "Permission" to the Mailbox (including "send mail").
Regards
Thomas