Squarespace doesn’t allow custom code via Code Blocks or Site Header Code Injection areas on personal plans, they are considered Premium Features.

If you’re on a personal plan, I have found a work-around that has worked for many years, but Squarespace might change this at any point. Use at your own risk.

The work-around is to add the installation <script> & <link> into a Markdown Block anywhere in one of your Footer sections (Edit Page » Edit Footer Sections).

Then, you can follow the instructions as normal.