You can create a tag and add it to a response in one action.
1) Go to the Feedback Tab in the Admin Dashboard
2) Find the response you would like to add a new tag to.
3) Type in the name of the new tag in the empty box with the + symbol. Click the + or press enter.
The tag you created will now be associated with that response as well as be added to the list of tags under the Tags Tab.
Under the Tags Tab you can see: