Tell you the truth, I don't have a clue how to do this in an input box, but you could call a modal dialog with a text box whose "Password Char" in properties in set to * (or any other character). You could also hide a text box and label on your exsting for and when your magic button is clicked, make them both visible and set focus to the password text box.
Not elegant, but quick and effective...
Let me know if this helps...
Best regards,
Jerry