The officially supported Matomo Connector for Looker Studio enables you to explore your Matomo data in Looker Studio at no cost. You can connect to both Matomo Cloud instances or On-Premise instances using just a Matomo Auth Token.
Looks like we can't even throw an error with a useful error message in this case. Looker Studio will simply swallow the error and say something along the lines of "the connector failed".
If it does not look like a URL we can report to the user via an error that the username should be a URL to a Matomo instance.
Might reduce some support requests.