The key needs to be in the ON position to supply voltage to the alternator for it to charge.
I don’t know specifics on your tractor but that is the first CW position on my old L185 then the next positions are headlight on then bright. When in the on position, before the engine is running the low oil pressure and low charge lights will be on. They then go out when the engine is running. But the tractor will still run when the key is in the off position, it just won’t be able to charge.