If you have Square set up as a payment processor, your users can go into their payments on file, located at /account/paymentonfile, to Register a Card for F&B Charges (e.g. Square POS).

Upon clicking this option, they will be prompted to add their card on file which will be synced to the Square POS.

You can now leverage this card on file directly from the POS by adding the customer to the order and clicking on payment where you will see this card on file.