I use a Url in a light box to send to a PARDOT form. In Pardot I then re-direct the thank you to our Thank you page which contains the GTM and records the completion that way. hope that helps.

Thanks for the reply. This is a smart workaround which I will use in other cases but not one that I can use in that case. The reason is because the formstack form I have embedded in ligthbox is also being used in other websites. Redirecting to the unbounce thank you page would be weird for the end user in some cases.

Once again, thank you for the idea, I will definitely use it in other campaigns where my form is only used in unbounce.

Pierre