I have made a small RSform Pro form that has identical fields compared to the custom fields used in AcyMailing. The form is filled in on the webpage, and when submitted it updates the AcyMailing subscriber's database correctly using a PHP script. Works smoothly. The users are not logged in (they do not have Joomla accounts). Now, when somebody already registered in the AcyMailing subscriber's list fills in the form again the data in the AcyMailing tables is updated correctly using the email address as a key. That too works fine. But I would like to use another field (member number) as the key instead of the email. If a member's email address changes it should be possible to report it through the form.

Has anyone done such an integration using a different key field and what PHP code is needed to accomplish that?

I haven't done something like that, but if you can find the correct string variable, it should be as simple as changing the string to get the id instead of email