I just checked and it seems that Apple watches cannot be used to record a reading of BP on their own....they can be linked to a regular cuff, maybe via an app.
My Samsung Galaxy watch, version 4, which I purchased in 2021, is capable of recording BP after a firmware update. I am reminded every 28 days (no idea why 28) through the Health App on my phone to re-calibrate the watch's sensor using an approved cuff device.
In case it is of interest to anyone reading in the future, a great time to get a solid read on your heart's health is just before rising in the morning. Prior to moving around and sitting up, use your smart watch to look at your waking pulse, and also your resting blood pressure. They should normally be in a narrow range, a range you will determine by keeping a record and running an average. Once you see a single morning's recording more than about 4% higher than the running average, it's a sign of an infection, inflammation, tissue damage, or overuse (meaning don't exercise that day...at all! Maybe not the next day either for extra insurance).