Honestly the transformer you have is not meant for the purpose you want to use it for. You would be better off getting something like https://www.grainger.com/product/ACME-ELECTRIC-Single-Phase-Transformer-4WUD8
Now I know what happend on Star Trek: Picard when the Romulans were clawing their eyes out. They saw that price!
If you need a real <= 5 KVA transformer, just hit Craigslist. They pop up there in the $100 neighborhood all the time.
OP, that thing you bought is not a "real" transformer, it's a Chinese cheater, and dangerous as all getout assuming water isn't around. I'm sure if water is around, that'll be fine somehow.
That wouldn't work. That transformer requires a 240 volt input....he has 120V.
Often, those cheaters are jumpable either way. A REAL transformer can simply be used in reverse.
The 240v plugs on this are European outlet and a weird "international combo" outlet.
Do you know how I'd attach that to wire?
Yes. Go to the hardware store and obtain a NEMA 6-15 socket, and a NEMA 6-15 plug. Make sure the 6-15 plug will actually fit in the "everything socket" - it really looks like it will.
Lop the existing cord about 1 foot from the generator plug end. On the pump side, install the 6-15 plug. Optional: On the generator side, install the 6-15 socket. Now you have the freedom to run it either way
Actually, the "International Combo" plug will allow a regular North American grounded plug to fit in it.
So now I need to find out how to make a whip to attach a North American plug to 10 gauge wire.
DON'T EVEN THINK ABOUT IT. Do not -- Do Not
-- put an ordinary NEMA 5-15 plug on that 240V load.
It's the same as wiping your butt with the Electrical Code, which I realize might only make you want to do it *more* lol. But you may get an expensive lesson if you do.