Create a custom property in your Hubspot account. Use the GTW registration as the zap trigger. Then add the update contact action for Hubspot and map the field to the newly created custom property. 

Thanks! How will this work if a person signs up for more than one webinar? Do I need to create a custom field in Hubspot for every webinar?

There may be a more elegant solution to this, and it also depends on how you run your webinars, but if people can only register for one webinar at a time, you can just overwrite the previous value. Otherwise, if they can register for multiple, you’d need to add however many custom properties as the max number of registration possibilities. BUT, that also means you’d need to do some kind of “look up” process to see which property is empty first, and then fill the existing registration into the first empty property. It’s pretty clunky, but it could be better depending on your webinar system and how you have Hubspot set up.

Thanks. I think this will be a messy solution so I will go with the standard emails from GoToWebinar instead of Hubspot emails.