My guess is that it works for certain airlines. I tried the three next flights fromthe local airport. Some of those are just one letter and three numbers. That makes it pretty hard to make assuption regarding the meaning of the code, without having a reference.
It's likely expecting IATA style flight numbers, which would all be two letters (or a letter/number) airline code, followed by some amount of numbers.
Seems a bit random -- QF1 works, QFA1 doesn't. SQ208 doesn't work, SIA208 does. But SQ21 works and SIA21 doesn't, so it's not even consistent within airlines. Might be related to whether or not the flight is currently active.
Ah, that is interesting, so it's taking both IATA and ICAO, but not consistently.