Steve,
It is easy to do that. When you are on working in your form design. Click on fields, and create a field name suitable to your liking(such as PageName, etc.) make it 300 text. In the default value click on the right arrow to open up the macro editor and place there. {% DocumentName %} exactly. Click save and you are all set.
You could get any value that a macro can resolve to, such as date, user, IP, etc. etc. as well as your own custom macros and use it as a default.
You could also use a macro to prepopulate information, for instance if you have members, you could prepopulate the first name, last name and email with the current user information and this saves them typing. If they type in something different then it will save the new values. Read up in the documentation about Macro's and Macro Syntax/Expressions. Also in the pop up window you can click the bottom bar to bring up available macro methods, sort of like intellisense. Very powerful.