From the Drafts Apple Watch documentation.
The Drafts Apple Watch app also supports direct integration with Siri, using the phrase “Create a note using Drafts”.
However I would not advise using your watch to do this while driving as it involves lifting your hand away from the controls to initiate and record. You also need to tap “done” at the end, and that’s a small tap target that would involve taking both hands off the controls.
Instead, I would suggest that you would be better using your phone. It works with the same phrase.
If you have recent air pods, consider wearing one of those. They respond to hey siri, and will give better quality in audio capture. But if you are able to position your phone well, I’m sure dictation without wouldn’t be too bad in any case.
Now, the triggered dictation isn’t the Drafts tricked out version that loops. But neither is the one triggered on the Drafts watch complication, and you don’t want to take your focus away from driving for too long.
Hope that helps.