Is it possible to make a landing page design in Photoshop for a customer who wants to use SquareSpace (website creator) after? It sounds strange to me. Thank you!
I would recommend reviewing customization options for available Square Space templates before going in the direction of custom design. If you get too creative in Photoshop, you might not be able to execute it on Square Space.
Keep in mind that Square Space, Wix and other companies offering free web authoring are essentially hosting companies, and make their money by tying clients to their own proprietary templates and platforms. They are ok for quickly deploying template-based sites but are not meant to support custom web projects.