I got "405 Method Not Allowed" error when I tried to unlock an account I created on MetaMask.
Here is the code:

    string privateKey = "0xXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
    string senderAddress = "0xXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
    
    var account = new Account(privateKey);
    var web3 = new Web3(account, "https://rinkeby.infura.io/cu9QSOeCCcK1lbKZrPey");
    
    var password = "xxxxxxx"; // where does this come from? the meta mask password???
    
    // error on the line below
    var unlockResult = await web3.Personal.UnlockAccount.SendRequestAsync(senderAddress, password, new HexBigInteger(120));

What have I done wrong? Where does 'password' come from?