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I apologize, but I'm afraid GoToMeeting was not designed for Outlook to choose between email accounts when scheduling. It should pull from the default or active email account in theory.
Another idea might be to right-click the GoToMeeting icon in your task bar, and select 'schedule meeting' in order to copy the invitation template into an email of your choosing.
Unfortunately the GoToMeeting calendar plugins only have the ability to use the current default address in the local application, as it will always use what's currently the default there.
Are you only talking about GoToMeeting, or some other GoTo services?
The reason I ask is because GoToMeeting does not send any emails to Attendees, as the Organizer is responsible for communications in this regard.
Your question (as far as I can tell) is one for Microsoft: https://support.office.com/en-us/article/Change-email-account-settings-58b62e89-6a9b-467b-8865-d5633...
I'm afraid you'll want to lean on Microsoft for support on selecting which email address is sent from. The only thing GoToMeeting is capable of is populating a template into a new meeting or appointment.