"onkeyup" is a property you can set to a function. Something like this:
function test()
{
alert("test!");
}
document.getElementById("yourtextfield").onkeyup = test;
That should do it, if you give your textfield the id "yourtextfield".
This page listst the event handler attributes you can set:
http://www.w3.org/TR/html4/interact/scripts.html