How Can I Assign Event Handlers Externaly?

I have written a script that gets trigered by IE's toolbar button. In
this script I would like to asign an event handler to an element of the
document currently open in IE. The way one access the document object
from a toolbar button script is:

var doc = external.menuArguments.document;

Now assuming the document has an element called TextArea1, the logical
thing to do would be:

parentwin.document.all('TextArea1').onkeypress = new
Function('window.alert('asdf');');

Which goes compiles and runs, except the event handler does not get
triggered.

Another trick i tried is as follows:

var s = parentwin.document.createElement('script');
s.text = 'window.alert('asdf')'
s.htmlFor = 'TextArea1'
s.event = 'onclick'
parentwin.document.scripts[0] = s;

Same result.