Hi @klas
Welcome to the Auth0 Community!
Thank you for posting your question. Unfortunately, it’s not possible to pass the URL parameter to the custom fields.
If that’s a feature that you would like to see in the Auth0, you can submit Product Feedback on our Community page to add more weight and visibility to the request, here is our link: Feedback - Auth0 Community. You can read our FAQ regarding submitting a feature request here: How do I submit Product feedback or feature requests?
Thanks
Dawid