I have the exact same question and tested a few things. Unfortunately, only the placeholder {Scheduler} worked. My custom event variables and {email} did not work.

You have to open following url in the browser:
https://url-to-selfhosted-calcom.domain.com/event-types/{YOUR_EVENT_ID}?tabName=advanced

There you have to click the "pencil"-Icon:

And you will see this variables: