Unless things changed, I don't think you can apply thru NS portal right. You either have access to their computers and apply yourself or you apply thru your commanders or the people handling leave issues. And even if can, instead of asking why don't you just go try it for yourself since you can't apply by asking.
Like what was mentioned previously, you need to apply for leave for fri to sun cause you can only go overseas when on leave but not off, but will only consume one day of your annual leave. But as far as I know again, unless things changed, the system doesn't allows you to start or end your leave on a non working day, so you should just have to apply for Friday and indicate in the remarks and also maybe give your commanders heads up that you will be overseas during that period. Anyway if you have to apply thru someone then problems solved...