Your manager as the co-host should be providing you with the copy of the contract s/he uses when he provides co-hosting services to his clients @Penny-And-Monty0 so this is where you should start.
Obviously check his references, that he has appropriate insurance and make sure the listing is under your profile NOT his so if you are not happy with his services you can easily use another provider or host yourself.
Make sure there is a clause regarding reasons for cancelling the contract and a notice period.