Don’t do either! (And don’t listen to ChatGPT without verifying, it’s an idiot.)
Set a system root password using the passwd command or login with a user that has sudoALL privileges. (The latter may be preferable.)
Using the Webmin changepass command is creating a new user just in Webmin, that is separate from the system user. This often confuses people months or years later when they forget they bifurcated their user into two independent creatures. It works, but there’s no reason to have a separate Webmin user database.