They request to volunteer it. 3 choices on the survey I had recently.
1. I am Vaccinated and have no problem showing proof.
2. I am Unvaccinated (I.e. Kids under 12 have to pick this)
3. I prefer not to answer
2 + 3 go with unvax rules. (Although kids under 12 do NOT have to get extra insurance nor pay for the test, but they still get tested)
The CDC Rules are a bit weird with Mixed brand vaccinations (AZ + Pfizer, Pfizer + Moderna, etc), which is causing bit of a stir though (Mostly for Canadians and others outside the country that did because that's what the government provided to them).
You can see the exact form here:
https://www.royalcaribbean.com/resources/vaccination-form