Passing variables to the src URL

I am currently working on getting our Metabase dashboards working in an iFrame in our CRM system. Showing the dashboards and passing simple filters to the source URL works just fine, but I am wondering if passing variables from the CRM system works with the source URL.

In our CRM I try create a variable, assign the ID of the current user to it and try to pass it to the source URL as $userID. But the filter just reads it as a string and passes it to the Metabase frame as #userID instead of the variable value.

Is it possible to pass variables to the source URL instead of just plain strings/numbers?

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if you're using public embeddings I think