250ml (250cc, 8 ounces, 1 cup, etc) of Dexron III or IV. Might be hard to find. I use Valvoline DEX/MERC and it works fine.
Do NOT use synthetic.
Let any excess drain out the overflow hole with the bike upright on level ground. The plug is in the 8 o'clock position when looking at the transmission cover.
I have a used engine that some moron put engine oil in instead of transmission fluid. I'm guessing that didn't work so hot.