How can I transfer eth from an account wallet to a smart contract

Question

I'm creating a smart contract that allows people to pay for a monthly subscription

I got stacked in this :

how to transfer plan.amount from the user wallet to the smart contract?

function subscribe(uint planId) external {
    Plan storage plan = plans[planId];
    require(plan.merchant != address(0), 'address not valid');

    bool sent = payable(address(this)).send(plan.amount);
        require(sent, "tx failed");
 
    emit PaymentSent(
      msg.sender, 
      plan.merchant,
      plan.amount, // the monthly amount for Subscription
      planId, 
      block.timestamp
    );


    subscriptions[msg.sender][planId] = Subscription(
      msg.sender,
      block.timestamp, 
      block.timestamp + 4 weeks // next payement
    );
    emit SubscriptionCreated(msg.sender, planId, block.timestamp);
  } 

Answer 1

The subscribe() function needs to use the payable modifier in order to accept ETH. Then you can validate how much the user has sent while invoking your function with the msg.value global variable.

It's not possible to request a specific amount from the contract, as the contract code is executed after the user has sent the transaction. You always need to validate how much has the user sent along with the transaction invoking the function.

function subscribe(uint planId) external payable {
    // revert if the sent value is not expected
    require(msg.value == 1 ether, "You need to send 1 ETH");
}

However you can control the predefined value on the UI, while creating the transaction request to their MetaMask or other wallet.

await window.ethereum.request(
    method: 'eth_sendTransaction',
    [
        from: userAddress,
        to: yourContract,
        data: <invoking the subscribe() function>,
        value: <1 ETH in wei, in hex>
    ]
);

Docs: https://docs.metamask.io/guide/ethereum-provider.html#ethereum-request-args