How do we do this on MS Health on w10m?

I've had that all along on 8.1 with the Health Vault App.

Health > Measurements > + > input your values.

With Health App, you could add a smart scale...

OK. I just opened Health, saw I had a new message (dot at upper right of Hamburger) > Hamburger > What's New? (which also showed a new message dot) Guess what? Track your weight. It is manual input, just like Health Vault.

Bob's your Uncle now, izzin't he?