That's a good one... I've played around with it a bit, and it seems that you need to use the same method as when loading a list of items to a list_box. However, it only works if you set both PopupEdit and SpinEdit to be true.
Unlike with a list_box, though, the result returned by this component is a plain string with either the selected item, or the user-entered value.
I'm looking for some syntax examples of how to load a list of items. I am using a plug in to design and write the code for custom dialogs. I have tried to find some examples of how to manualy write code to establish the items in the dropdown but am not finding anything. I must not be looking in the right places. The plug in that I have writes the bulk of the code for the dialog but there are oportunities to add additional action code inside the plug in and I'm still trying to figure it out.
Check out example #3 in the API reference of execDialog.