The easiest way is to use the CASE number as unique ID:
This is your ID: %caseNumber%
Of course, that's quite easy to "hack", i.e., someone might just try different numbers.
So, you could also ass some confusion:
This is your ID: 3%caseNumber%472
Or if your respondents have a SERIAL, you may also use that:
This is your ID: %caseSerial%
And then you can also use completely differen IDs: Display individual codes or voucher codes