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

You can add meta tags 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 <meta>. Screenshot:

After that you will need to give a name to the code that you're placing and then fill in the fields based on your meta tag. Here, for example, I placed a meta tag that prevents the site from being indexed:

Before, this meta tag looked like this: <meta name="robots" content="noindex">