How to build & sign a raw transaction calling a function on an 3r party already deployed contract

Question

My goal is to sign a raw transaction, then broadcast it through the Ftmscan/Etherscan API. I don't want to deal with web3 providers like Infura/Alchemy, if possible. Just write the raw transaction, then FTMScan API will broadcast.

I understood how to create and sign a simple transaction with Node and the Ethers.js library :

const ethers = require('ethers');
async function main() {
// defining the wallet private key
  let privatekey = 'my_wallet_private_key';
  let wallet = new ethers.Wallet(privatekey);
// print the wallet address
  console.log('Using wallet address ' + wallet.address);
let transaction = {
    to: 'my_wallet_adress',
    value: ethers.utils.parseEther('0'),
    gasLimit: '200000',
    maxPriorityFeePerGas: ethers.utils.parseUnits('100', 'gwei'),
    maxFeePerGas: ethers.utils.parseUnits('115', 'gwei'),
    nonce: 0,
    type: 2,
    chainId: 250
  };
// sign and serialize the transaction 
  let rawTransaction = await wallet.signTransaction(transaction).then(ethers.utils.serializeTransaction(transaction));
// print the raw transaction hash
  console.log('Raw txhash string ' + rawTransaction);
}
main();

Ok, not difficult. And it works when I broadcast it with the FTMScan API.

But I don't understand how to interact with an already existing contract.

For exemple, let's take the "mint" function (= deposit on lending pool) on Impermax (https://fantom.impermax.finance) : Contract : https://ftmscan.com/address/0xb9f3413e206f1d658d4dafb233873dde56cf94fc#code

When I execute a deposit from the UI, and from what I can read in Ftmscan, I can see this function is waiting for 4 parameters :

function mint(address poolToken, uint amount, address to, uint deadline) external returns (uint tokens);

How can I insert this function call with parameters in the above transaction ? Is it possible this way actualy ?