You can pass a List of String or Dictionary to populate the dropdown. You bind the collection to the dropdown control by using [ControlName]_dropdown. The [ControlName] of a dropdown is the Property Name within the Field Key tab. This would be referenced within the FormFieldsCollection property under Data Bindings within the Create Form activity.
Please take a look at the sample code which is built in 19.10.1.
CustomForm_DropdownInput_Example.zip (20.2 KB)