Is it possible to customize (add branding to) the page that says "Connected to GoToWebinar.... Let's get started!". I believe it's the landing page once you register for a webinar. I know you can somewhat customize the registration page but this is the page before the webinar starts.
Although the GoToWebinar connecting messages cannot be altered at this time, there is one customization you might be interested in, the post-registration page redirect. This functionality allows you to send your registrants to a web page of your choosing, and is customized on the individual webinar schedule online in the Registration setings.