I want to add tags to existing contacts based on a form

Hi,

I want my contacts to fill out a form so I can add tags based on their answer.
Example: Do you like A) Apples or B) Bananas’.
So when they choose A, they get tagged with A.
How do I do that without them having to optin again?
I know I can do it during the optin, but i don’t want the friction.

Any help is much appreciated.