Nothing you enter in the FastWeb questionnaire is “binding." This means that you can indicate that you are a military spouse and then see what comes up. However, when it's time to actually apply for a scholarship and to receive one, you will have to be already legally married and able to prove it, if the scholarship is intended for military wives or husbands.
But, for now, go ahead and call yourself a spouse just to see what options could be available and what you would have to do to get them. But, in doing so, you must clearly understand that you cannot apply for these scholarships until you are officially married. Depending on the timeline (when the scholarship winner will be determined and when you expect to be wed), you may—or may not—be able to go ahead and file a scholarship application once you've identified the possibilities.
Happy hunting and best wishes to you on your engagement!