Converting AS2 to AS3 help

I have AS2 code that looks like this

TextField.prototype.onKillFocus = function()

I want to convert this to AS3 code using an eventListener. I tried using

TextField.prototype.addEventListener(FocusEvent.FOCUS_IN, setProps);

but it doesnt work. Could someone please help me in figuring out how to make a generic function that can be used for multiple TextFields? Thanks