If you want to control the drop-down menu during runtime, create the drop-down menu programmatically.
Refer here for more information about adding a drop-down menu.
The property 'Items' can be used to control what all to be listed in the drop-down menu.
Also, the 'Parent' property can be used to control where the drop-down menu needs to be placed. The position can be further finetuned by adjusting the 'Position' property.