Insert File Path into Text Field

I am trying to create a text filed at the bottom of a form document that will automatically input the file path (e.g.. C\:Folder\File) of the document itself after the document is saved.  I've looked at a few Scripts but nothing seems to be working.  I'm assuming it will look something like this on the "postSave" option:

this.rawValue = event.target.path.replace(event.target.documentFileName, "");

I am a very novice programmer so I have no idea if this is even close to right.