John, It would require a bit of a client scripting.
I guess the easiest way would to wrap you text box into <div>
tag with some id (for ex. mytexboxdiv) and some javascript that sets value to your text box:
if (window.parent != null) {
document.getElementById('mytexboxdiv').getElementsByTagName('input')[0].value = window.parent.window.location.href;
}
HTML Envelope Content before:
<div id="mytexboxdiv">
HTML Envelope Content after:
![Image Text](https://image.prntscr.com/image/f6s1IbgZTaO0GrHmaRNueA.png)
P.S. the editor does not let me paste script here