We need this as well.
I tried using the code but have found that the contact/form information is not consistently making its way to Hubspot nor are we getting notified of the form submission. The redirect is happening faster than the info is getting into Hubspot and then it’s getting lost.
Hubspot suggested changing onFormSubmit to onFormSubmitted, but that didn’t redirect at all.
We’ve gone back to our original solution, which was using PlusThis SmartLinks.