Set Experience Cloud ID (mid) to a eVar in Processing Rules

We would like to set "mid" to a eVar via Processing Rule.

On Stackoverflow there is a reply that says:
"Well PRs can populate variables based on query parameters in the collection url, so you can just skip the javascript coding and map mid query param directly to your eVar or prop.

https://stackoverflow.com/questions/46778954/how-do-i-store-a-adobe-marketing-cloud-id-in-an-evar

But it does not say what  we should use in PR to get the data from. I tried "Query String Parameter" (assuming it would not work).

But don't get any data.

Anybody knows what to use, or how to do it.