Trigger A Function From Pop Up Window And Arguments

I have a main page that opens a pop up window when the user clicks a
link. The pop up window is a menu and when the user clicks the item, it
populates a form input element on the 'parent' window.

My question is: this new populated form element is the value that I use
to change a style.

onchange does not watch this form element because of getelementbyid, so
it doesn't trigger the function needed to change the style. onchange
works fine if I just manually input a value into the text box.

Any other way to fire the function off when the user picks their
selection from the pop-up window? I was thinking I could use
window.opener.function()? But how do I pass the form from the parent
window? From the pop up window, I don't use this.form, what do I
use?something like window.opener.form[0] ?