You build the form within Google Docs like you normally would – a text box with a label, etc.
You then determine the ID of the field (it likely will be something like entry.NNNNNNNNN where NNNNNN is a string of numbers) as you need the ID to define the field as hidden when defining it within WordPress. When you define the hidden field there are a number of preset values it can take on based on your site, the user’s login name is one of them.
There are pictures in the post on my web site I pointed to previously.
You do not need to set the value to anything when defining the form within Google Docs.