If it's not there, you can use php to grab the value directly from the DB.
Also consider testing by putting a sample page in your web_form or web_form 2 fields and "dumping" the URL to see if the value you seek appears anywhere. These two fields will generate ALL available fields if not specifically told to only generate certain ones. So put in a php page, and use PHP to dump the contents to the screen (or just copy the url generated and paste it into a text editor, split into lines at "&" for ease of reading).
Beware of pushing the web form button when there is no entry, it will push to a sourceforge.net page that you do not own and WILL push the username AND PASSWORD of the agent to that site.
If not, then you could consider paying to have it added (The Vicidial Group or a 3rd party developer like poundteam) or adding it yourself or ... grab it from the DB directly when you need it. Grabbing from the DB is powerful, as then you can actually grab anything you want. PHP is a marvelous tool.