Hi Arthur,
Thanks for the request, it is currently not possible to set a redirect URL on submission so that once a survey is submitted in web app it automatically redirects to a different website instead of the thank you screen.
Currently the best workaround for this is to put the URL of the website you want to users to be taken to into the thank you screen, and an instruction for them to click the link.
If you would like to see this feature added as an enhancement in the future, I strongly recommend you raise this as an enhancement request with Esri Support and also create an Idea on the ArcGIS Ideas page for Survey123, so other users can also vote for this request.
Regards,
Phil.