In your Squarespace backend go to your home/config page. Click on Settings -> Advanced -> Code Injection Now copy and paste the code given to you by GTM into the following locations. You can use the icon to automatically copy the complete code. Copy the code and paste it into the first code input field labeled "HEADER" on the Code Injection page on Squarespace. Copy the code and paste it into the second code input field labeled "FOOTER" on the Code Injection page on Squarespace. Hit "Save" to update the changes you have made.