This post is not an issue but something I experienced
In an email there is a link to an external app without link with marketo (here typeform for example)
I wanted to know who had fill out the poll on typeform with a confirmation page on my website for send to them another email
The problem was there is no cookie from Marketo on my Typeform poll so it was impossible to connect the lead who had clicked with my confirmation page (because confirmation page = typeform filled out)
So for solve it, I added a doorway page before the typeform on my website to deposit the cookie
And it works!
You didn't need to do that for Typeform, though. You can pass the Marketo token as a hidden field to and from Typeform, so the Thank You URL (on your website) gets associated with the lead.