Citrix Online has just made public official APIs for GoToMeeting, GoToWebinar and GoToTraining, more information can be found here https://developer.citrixonline.com/
I’ve been sitting on this one for quite a while, mainly because it’s not something officially supported by Citrix Online and the team at GoToWebinar. But if you know a thing or two about putting a webpage together you can download this example to build your own custom GoToWebinar Registration page!
All credit for this example goes to Mike G. a brilliant former co-worker. Good on ya mate!