Please give this manual a look:
Working with External Panel Providers
There is a lot of information how to transfer information through a redirect.
What you need to do is (a) put the case ID (e.g.
%caseNumber%) in the redirect to the online experiment, make sure that the experiment takes and stores that code, and that it is, again, put in the redirect to the post-survey.
You can either use the "r" variable in the URL, or you can use a "device and transfered variables" question on the first page, to take an store the code.