Code from https://www.freecodecamp.org/news/how-to-add-custom-network-to-metamask/

wallet_addEthereumChain.js

window.ethereum.request({
  method: "wallet_addEthereumChain",
  params: [{
    chainId: "0x89",
    rpcUrls: ["https://polygon-rpc.com/"],
    chainName: "Matic Mainnet",
    nativeCurrency: {
      name: "MATIC",
      symbol: "MATIC",
      decimals: 18
    },
    blockExplorerUrls: ["https://polygonscan.com/"]
  }]
});