It seems like you need some experience binding data to Telerik's RadControls, namely the RadAutoCompleteBox. Hopefully the documentation online
can give you a further insight into how data gets bound to the control object. If you are trying to narrow down data from larger (Region) to smaller (City), you will need to write a function under the dataAdded event from the parent AutoCompleteBox to successfully populate a child AutoCompleteBox's entries. You may find some insight here
on how to implement this.