Unable to ad xyz.stage.lithium.com to allowed domains for http.client calls

Hi all

We've built some custom endpoints to aggreate some data from the API. Now we'd like to make another endpoint that aggregates the data from the custom endpoints. 

As far as I've understood I need to make the call using the http.client.request method which in turn requires me to add the allowed domains in the Administaror. As our clients stage is currently running under xyz.stage.lithium.com I tried to add this domain to the field, but that does not work. I can add other domains, but something with lithium.com just won't be accepted. 

Do I really need to get a CNAME for this stage instance so I can access it by something.myclient.com?

Thanks!