That's your starter relay. It goes between the positive on your battery and the starter. Your handlebar start switch controls it. You're still going to need a regulator/rectifier for charging. BTW, do you have the 2k2 or 2k3 stator? You'll need the bigger one for charging a battery IIRC.
Sorry, but it was the 2015 manual I used for wiring. Here is a download link:
https://drive.google.com/file/d/1h9-...ew?usp=sharing I think it's page 21 you need. Make sure the schematic is listed for the 250/300 e-start models.