Indy 9.0.3b - How to select a different network device for Multicast ?

Seems I found the error in my Multicast Client application.
I tried it on a machine with only one network interface -> works
Adding a second interface -> doesn't work anymore, as it seems to send to
the wrong interface
But how to set the send-to-interface ? I tried to set the binding, but this
didn't work...
Any hint on this ?