Hi ihm,
Very similar situation for me from years.
Our web site is another domain as Adobe Campaign webApps, and many restricting CORS and so on.
We can't manipulate easily Javascript/html open windows parameters (_parent etc).
For example, to enforce the parent page (the website), to change the iframe size or scrollbars under events.
I tried with standard End activity and Javascript httpRequest redirection.
The easiest solution was to redirect to a thank you page managed by the web site, instead of a page of the webApp, and pass it some parameters for the web site instructions.
Probably someone in the community could indicate us some workarounds.
Regards
J-Serge
