After you have created a blank page, click on the page from the left side menu

On the top left of the page, click edit

Squarespace will prompt you to "add section"

After a section is created, you'll see "add block" button once you're hovering over that section

Type in the search field "code" and select that option

Once it's added to the section, click on the pencil symbol to edit the code

Paste the buy button code into the text editor and ensure that it says HTML on the drop down above (not css/javascript/markdown/plain text)

Ignore the warning msg containing "this block contains embedded scripts. embedded scripts are disabled while you're logged in and editing your site"

Once done, click on "done" button at the top left of the page, then click save

Your buy button should now display!