You can add embed code to individual posts in Squarespace and to the menus on your home page.
From your Squarespace dashboard, open the webpage where you want to add the embed code(Checkout Button Code).
Find the element on the page where you want the Checkout button, move your cursor over the Page Content area, and then click EDIT.
Click the insert point where you want to add the embed code(Checkout Button Code).
In the More section of the Content Blocks dialog, click Code.
From your Create section, copy the Checkout Button Code from the COPY SCRIPT pop up.
From your Squarespace dashboard, paste the embed code(Checkout Button Code) into the CODE dialog. Make sure that the text field is set to receive HTML.
Click APPLY on the EDIT CODE dialog.