How to send BNB (or other networks) with web3 on a dapp?

Question

So, I tried the many examples found around but I never manage to get it working.

I have this snippet:

async function pay(cost) {
        code = 0
        message = "Please approve the tx"
        if (typeof address == 'undefined') {
            alert("Please connect to your metamask.")
        }
        const paymentAddress = 'MY_ADDRESS'
        const amountEth = cost.toString()
        var web3 = new Web3(Web3.givenProvider)
        res = web3.eth.sendTransaction({
          from: address,
          to: paymentAddress,
          value: web3.utils.toWei(amountEth, 'ether')
        }, (err, transactionId) => {
          if  (err) {
            console.log('Payment failed', err)
            code =-1
            message = "Failed"
            return message;
          } else {
            console.log('Payment submitted', transactionId)
            code =1
            message = transactionId
            return message;
          }
        })
    return res;
}

Which is working great in requesting a value in ETH when connected to the ETH Mainnet.

My dapp should be able to request BNB if connected to BSC or FTM if connected to Fantom.

I managed to create a connection thus i have the address, chainid and so on. I tried simply changing the cost, it doesn't work.

How one can do this?