HERE'S THE SOLUTION:
In Google Analytics create a new property.
In the new property setup panel there's a link "Show Advanced Options" at the bottom.
In Advanced Options there's an option "Create a Universal Analytics property". Using this option will give you the option to either create both a GA4 and a UA id in which case it creates 2 properties and links them or you can just create a single UA id.
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".