Setting up a signer

You'll need access to a signer object to send a transaction to a Squid contract.
More about signers: official ethers documentation.
Setting up a signer in node
// example setting up a signer using Ethers (please check their documentation)
import { ethers } from "ethers";
// note: DO NOT push your private key to a git repository or anywhere public
const privateKey = "<YOUR_PRIVATE_KEY>";
const ethRpcEndPoint = "<YOUR_INFURA_PROJECT_ID>";
(async () => {
const provider = new ethers.providers.JsonRpcProvider(ethRpcEndPoint);
const signer = new ethers.Wallet(privateKey, provider);
// rest of your code