[Ethereum] How to create a MetaMask Connect request to auth a wallet with MetaMask

Check out this guide. To summarize: first check whether web3 already exists. If it does, initialize a Web3 object with web3.currentProvider. Otherwise, use your node's URL

The Metamask interface has changed to enable privacy and consent from the user before allowing any access to the account information contained within Metamask. This is done by injecting an "ethereum" object to the browser window. You must now wait for the ethereum.enable() function to return true after prompting the user. More details here: https://medium.com/metamask/https-medium-com-metamask-breaking-change-injecting-web3-7722797916a8

window.addEventListener('load', () => {
 // Wait for loading completion to avoid race conditions with web3 injection timing.
  if (window.ethereum) {
    const web3 = new Web3(window.ethereum);
    try {
      // Request account access if needed
      await window.ethereum.enable();
      // Accounts now exposed
      return web3;
    } catch (error) {
      console.error(error);
    }
  }
  // Legacy dapp browsers...
  else if (window.web3) {
    // Use MetaMask/Mist's provider.
    const web3 = window.web3;
    console.log('Injected web3 detected.');
    return web3;
  }
  // Fallback to localhost; use dev console port by default...
  else {
    const provider = new Web3.providers.HttpProvider('http://127.0.0.1:9545');
    const web3 = new Web3(provider);
    console.log('No web3 instance injected, using Local web3.');
    return web3;
  }
});