Select the Text you will be working with and in the Data Source tab, ensure that Static Text is chosen. Then select the Enable User Input Controls checkbox.
The Combobox will be a dropdown, but there are other options here as well. For instance, Radio Buttons and Check Boxes.
In the User Control section of the Data Source tab, select the Static List Option and then hit the Add button. In the Description window above you will see a field that is highlighted and reads New.
Enter the desired text here. Use this for as many items as you'd like in your dropdown.
The final Result should look like this:
