Here's how to solve it:
Login to you Sqaurespace website. Then Settings >> Advanced >> Code Injection >> Header. Here is where you add the new Global Site Tag "G-" code. Click SAVE.
To get the new Global Site Tag, log into you Google analytics account. Go to: Settings >> Property >> Data Streams. Set up a Web Stream for your website (unless you already have it). Then go to Tagging Instructions and copy the Global Site Tag (gtag.js) code. This is the code you have to paste in Squarespace Code Injection "Header".