Allow registered user to assign additional category to post from front-end

I’m interested in allowing registered users to assign an additional category to a post they’re viewing by clicking a link or button.

The category would already exist. There would be no need to choose the specific category to assign (that would be done already). All they need to do is click the button to add the category.

I know this is possible by going to the dashboard and editing the post and assigning a new category. But for my purposes, I would need to stay on the front-end.

Does anyone know how this could be done?