Writing A Variable To An Other Document

i have a page called 'a.php'. on this page is a form with a textfield and a link.

if you click the link an pop up opens. in this pop up is an number visible with a link.

what i want is this:
when you click the link in the pop up, the number from the pop up must appear in the textfield from page a.php.

i tried:

a javascript under the link in the pop up with:

function brinnummer(nummer){
document[0].getElementById('brinnummer')[0].value = nummer;
}

but it doesnot work.