Combo boxes don't all work the same way. Ideally you can spy the element as a combo box and use a Navigate to select the item you want. You may also be able to spy individual items in the list.
Or, as described above, you can focus it with a mouse click, type the value you want and the combo box will select your item. But if none of these approaches will work, then you may have to take the most rudimentary approach and create a loop to send Down keys until you reach the right selection. This can be difficult to perfect and slow to execute, but sometimes (eg with Surface Automation) it is the only way.