Custom connector not using Host URL
If anyone else comes across this post, here is the answer.
If everything is configured correctly, unfortunately the Host Url is not displayed in the request field but always an MS Url. And yet the Host URL is used to retrieve data from the API. It is therefore important to configure everything correctly and not to be confused by the displayed URL in the test request field. After I had done this, correct host, which is displayed under Definition, the correct API key and the correct magic words (keywords), then everything works fine.