How Do I Add the <Link> tag into <Head> on All Pages?

You can add any plugin code for the whole site in your project settings. To do this, click on the "Arrow" icon next to your project name and select Settings. Then go to the "Custom Code" tab, click on the "+" icon, and select "Link". Screenshot:

After that you will need to give a name to this link and fill in the necessary fields based on the URL you want to plug in. For example, I plugged in Google Fonts in the screenshot:

Before, this code looked like this:
<link rel="preconnect" href="https://fonts.googleapis.com">