Unfortunately it is still disabled on eu-latest to appear in the UI selection list (the key is only to make it visible).
Still, you can and should be able to add your passthrough endpoint using REST. How does your config look like created via REST?
In this guide it is explained how to add the endpoint using REST.
cumulocity-remote-access-cloud-http-proxy
Did you follow this instruction:
The name of the configuration should be prefixed with either
http:orhttps:depending on the server you are trying to connect to. This is used by the UI Plugin to identify endpoints that are compatible with it.
In case the tab does not show up, check the browsers console logs. In case you’ve missed to set a tenant option, you should see a warning there.