Change Form Field Value In New Window

What I need to do, is to change a form field value in a new window (window2) based on input from the parent window. The new window (window2) will pull up an existing site, with a form, which I can't change the code on. I will only be able to work with the code on the parent window.

After I load the new window (window2), with the onload function, the error says: ('window2' is undefined).

For example, here is the basic code I have been trying to work with:

<script type="text/javascript">
window.
window.open("window2.htm", "window2");
window2.theform.field.value=1;
}
</script>