Support for custom group list?

I would like to use a custom package group list and I can see that there a API endpoints in katello and pulp3. But I cannot find any actions for it in th web gui.
When I try the katello API endpoint I get an error:

curl -k -H "Accept:application/json" -H "Content-Type:application/json" -X POST -u admin -d '{"repository_id": "61", "name": "PKGs", "mandatory_package_names": ["jq"]}' https://$(hostname -f)/katello/api/v2/package_groups
{"displayMessage":"key not found: :content_type","errors":["key not found: :content_type"]}

Also uploading a custom comps.xml by using the web gui failed with error “RPM file cannot be parsed for metadata”.
Has anyone a suggestion how to do it?
Kind regards