Depends on the control used. Assuming a regular text field, character code 127 (0x7f), an obsolete mapping for the DEL control code. It is displayed as a rectangle since there is no Unicode character for it. This will not necessarily be reproducible; the Rich Edit control will interpret it as the “delete to begin of line” control key. It is very unlikely that you will be able to enter such a password at the invitation of a web page, for example.