Adding a drop-down list to a form

Sorry, I'm sure this is a dumb question, but it's stumping me...

I've created an online customer form that users can fill out. To add fields to it, I go to Setup -> Marketing -> Online Customer Forms and then click on the Select Fields tab.

I'd like to add a custom drop-down list to this form but I can't see how to do it. The fields already seem to exist. Is there a way to add another field that isn't already one of the options?

Thanks so much!
~emily