If your combo box is not supported, you might be able to get all the items in the list by first dropping down the combo box then using FindAllChildren to get the items in the list. You would need to find them using a property like "MenuItem" that matches their type. Once you got them back in an array you could verify what you got, or get the index of the items and then use it to click them. To click them you would likely need to dropdown the menu again.
Here is a link to the help topic on FindAllChildren: FindAllChildren Method | TestComplete Documentation (smartbear.com)
