When filling out the G-325A what do you put in the "file number" section. I have not submitted any paperwork yet.

I am currently stationed overseas in with the U.S Army in Vicenza, Italy. I got married last November and am trying to have my wife come with me to the states. I will be separating from the military in about 40 days so I am trying to get her paperwork submitted before I go. I have yet to file any paperwork but I have completed the I-130 and have all supporting documents. Where I am confused is the G-325A when it asks for a file number. Do I leave that blank due to the fact that I have yet to file any paperwork or is there a place I can go to get issued this number.