Assigning Button 1 on Saitek yoke to talk to ATC with Radio Contact

Could anyone please advise how I can assign button 1 on my Saitek yoke so that when I hold it in, I can talk to ATC using Radar Contact 4. I don't mean bring up the ATC menu and then start pressing numbers. I mean hold button 1 down and speak through headphone mike and release when finished communicating. VOX ATC, Radar Contact etc allow this type of realistic spoken ATC communication, but I cant seem to assign a button on my yoke to open the channel.

Any advice from someone who has done this successfully already would be most gratefully received.

Thanks in anticipation

Andrew