Use user input in Combo Box to perform a Definition Query

I'm nut sure which function (i'm assuming onEnter) to use and how to write the code to be able to get a users input ( type it in the text box of the combo box), hit enter and have it performe a definition query on a layer. I can accomplish this by using a search cursor and populating the combo box with the returned values, but i prefer no to do this as returns a rather large list. Any thoughts?