How to pass a custom string from page to page

I have searched endlessly to find an answer to the following but to no avail, so am hoping someone could assist.

I want to pass a custom string in the URL (?Refer=) from page to page on my wordpress website (www.b12leads.com) so that where ever the user goes the value is maintained in the url.

My hope is to use this string to customize pages and record from where the user was referred at a later date.

At this point though all that is required is to maintain this string in the url when a user navigates around my site.

Example

Main site = https://www.b12leads.com

External to the site I would provide different links, https://www.b12leads.com/?Refer=test1, https://www.b12leads.com/?Refer=test2 etc

How do maintain the “?Refer=test2” as the user goes to different pages or posts?

All help appreciated