Adding tags based on answers in form

Here is my question:
I want tags to be added to a contact based on the answers in one form.
If contact clicks checkbox x, tag a and b are applied.
If contact clicks checkbox y, tags c and d are applied.

I found this: How to add different tags to a contact based on an answer choice in a check box list on a form
But I can’t figure it out. I can’t find the “field changes” action.

Thank you for your help!