How do I make a Textbox Postback on KeyUp

Question

i gave one dropdown which name is ddlproduct and one textbox name txtqunatity

 

when the ddlproduct.selectedvalue and txtquantity send to database then it return some msg 

but to that return only  when ddlprodcut set to postback=true but i want to do this on when the value is insert in textbox then it return the result