Does it have to be on a smart trainer though? I trialed it a long time ago (4/5 years) and manged to race using my cadence sensor and imputing my own stats. I guess it was far more crude back then with far fewer people with smart trainers.
No, you can use a dumb trainer, and it uses an algorithm to figure out your speed etc. Some race organisers won't allow you to get a result, but apart from that you'll have no issues.
@Disgruntledgoat this kind of showing off is common (from the Zwift FB page today)
Knobbers.