Thanks for the examples, Joffm. I think this is the best way to approach the problem, but I need to host the codes on a separate server to ensure privacy. Can you suggest a simple way to set that up?

Hi,

well, my Limesurvey installation is on " www.example1.com/survey " and the phpfile is on " www.example2.com/mydirectory/getCode.php ".

To be more sophisticated:
You can place the codes encrypted in a database.
Change the php to connect to the database, get the code, decrypt it....

So it's up to you to adapt it to your needs.
I just gave a simple example.

Best regards
Joffm
Joffm