Rather than adding a dropdown list, dropdown lists work by taking an existing text field and converting it to a dropdown. So, first add a text box from the menu you have in your screenshot, and then convert it using the menu two to the left, with this icon Image may be NSFW.
Clik here to view.
. This is also how you create radio buttons, by the way.
During the process of conversion you specify the labels and values that will appear in the dropdown. This is a static list and cannot be generated dynamically. For each entry the label is what appears on the screen and the value is what you get when you copy the field via a script.
Steve.
