Well, we did something like that very recently, but keep in mind that I am not an electrician at all. I went to Lowe's and got an appliance cord like this one:
https://www.lowes.com/pd/Range-Hood-Power-Cord-3-1-2Ft-115V-Incl/50360502 and hooked it up to our new hood using the wire nuts. This way we were able to test it before installation.
We were replacing an over the range microwave that plugged in, though, so it made sense for us to get the cord since it would be used permanently to power the new hood. To do it for free, I don't know, maybe you could scavenge an appliance cord from something else... whatever you do, you'd need an appliance cord with a voltage greater or equal to that used by your vent hood, I think.
Hopefully somebody on the forum with actual electrical know-how will weigh in with any other precautions necessary.