Squarespace Commerce APIs

In a Squarespace merchant site on the Commerce Advanced plan, log in and go to Settings → Advanced → Developer API Keys, click GENERATE KEY, choose permissions, and save the key when shown; Squarespace says this is the only time the key is visible. See Authentication and permissions.

To build a Squarespace Extension, submit the registration form linked from the docs with your client name, icon, redirect URI(s), initiate URL (for partner apps), terms URL, and privacy URL. After review, Squarespace provides client_id and client_secret; then use the OAuth flow in the docs to obtain access and refresh tokens. See Authentication and permissions and Accessing Squarespace APIs using OAuth 2.0.