Setting AutoCompleteExtender ContextKey from Ajax CascadingDropDown

Hi folks, Need help on setting AutoComplete ContextKey using cascaded dropdownlist.

Here is the scenario. I have two dropdownlist ( + Ajax  Cascaded DDL) & TextBox( +  AutoComplete)

DropDownList1- To Select Vehicle Type ( Air, Bus, Train)

DropDownList2-  To Select Vehicle Category (Like If Air-Economy, Bussiness Class. If Bus - A/c, Non A/c, If Train - 1Ac, 2AC, ChairCar etc)

Base on this 2 DDL selection i need to set context key of TextBox (Vehicle Name IF Air - AirIndia, Indigo, AirFrance..etc which user will be typing ) to filterout the records. 

This can be achieved by making DDL autopostback=True and Using DDL selectindexchanged events. But what i need is 

Is their a way to set AutoComplete ContextKey through CascadedDropDown selected index change. Cause i really don't need a postback to set the context key. any idea Guys......:)