Mapping URL parameter to attribute

I need a way to pass along the Google ID (gclid) from a URL to our CRM. When I filter conversations by gclid url parameter I can see that conversations are coming through. How can I get this information passed to our CRM.

I thought I could create a custom attribute called gclid (which I did) and then map the URL parameter to that attribute, so I could then map it to a field in our CRM. I can't figure out how to get a URL parameter to write to a custom attribute field in Drift though.