Write code that can capture, carry & forward a unique id code (wordpress)

I need a simple code that can capture, carry & forward a unique id.

I have a wordpress website or two.

I want a little bit of code added to the sites that can capture a unique id code that a visitor uses.

mywebsite.com/page?id=unique - I want the ?id=unique part to be captured by the site (read) then ensure it is past on to every page from this point forward. I also want it to be sent as the visitor leaves.

I understand this is like a affiliate program and that there are hundreds out there etc.

I don't need to track or record the stats of these ids.

I simply need the function.

Here is an example of a visitor and how the code functions

Visitor clicks a link with an id on it.
mywebsite.com/content-page?id=unique

visitor then clicks a link that to another page and id is carried forward.
mywebsite.com/video-page?id=unique

visitor then exits site with the code.
signupsite.com/join?id=unique