Bruce is right that FS usually auto assigns a squawk code of 2200 (which is fine) however, Don't squawk 1200 unless you filed a VFR flight plan. If a controller signs on while you are inflight, they will assgin you another squawk code appropriate to their sector.

But please don't just leave any squawk code in the box, like for the previous flight you had, as it can cause conflits with operating ATC's who might assign the same code to another a/c close by. Always use 2200 when no ATC is online to assign a code.