Hi,
I am fairly new to limesurvey and don't quite get what the best solution for my idea is.

My questionnaire consists of 2 parts. After part 1, participants should be redirected to an experimental task and as soon as it is finished linked back to the questionnaire where they are supposed to fill out part 2.

So far I have created a new category with text that links to the site: domain.com/exp?id={SAVEDID}

Secondly I'd like to disable the possibility to click continue before coming back from the ext. site. I have tried to create a hidden question called exp that has to be answered and modify the backlink accordingly: domain.com/limesurvey/1?exp=1
Doesn't seem to work as intended though.

Finally do I need to transmit the token as well {TOKEN:TOKEN} or is it sufficient to allow participants to continue later (I guess it will work with cookies then).

Thank you for any help on what's the best way to do this.

you could do it with an hidden question that performs an ajax request to the 2nd web site to know if the task has been done or not.

How about using two surveys, redirecting to the external task with the End URL of the first survey?

I think I will use 2 surveys. Can I link out with "?id={SAVEDID}" and then use this id for the second survey?

Yes, that should work.