Donors will click on the call-to-action in their email and be brought to the wiki to create an account, hopefully to go on to edit. We want to be able to analyze the behavior of this cohort in all the ways that we usually analyze Growth newcomers through EventLogging and edit tags -- but we want to be able to separate out this cohort to see them distinctly.
We'll do this through identifying a URL query parameter in the call-to-action link. We may want to keep track of multiple query parameters if there are different subgroups in the cohort (e.g. new donors / repeat donors) or if we try different versions of the email. For example, the call to action link might be to `https://es.wikipedia.org/wiki/Special:CreateAccount?campaign=growth-mexico-2021-a` for variant A of an email or `https://es.wikipedia.org/wiki/Special:CreateAccount?campaign=growth-mexico-2021-b` for variant B, for an email campaign sent to donors in Mexico.