Getting field value from child window [Using Redbox]

This is javascript question. Imagine your textbox have id 'blabla'.

In your child window, your submit button must invoke this javascript
question:
top.opener.window.document.getElementById('blabla').value =
the_new_value;
window.close();

then you can just use
window.document.getElementById('blabla').value =
the_new_value;

I don't know the implementation of redbox. I assume redbox make "modal
window" not using child window. So redbox page is same as in parent
page.