To deposit money into a Metamask gambling site, you must first have Ethereum (ETH) in your Metamask wallet. Then log in to the chosen gambling site and go to the deposit section. Select Ethereum as your preferred payment method.
The site will provide you with a deposit address (a cryptographic code or public key). Copy this address and paste it into your Metamask wallet, then enter your desired amount of ETH. Confirm the transaction and the funds will be deposited into your gambling account, ready for betting.