Get the official endorsement from an SQ ticket office.

(I was in the reverse situation: used my credit card to book sq.com for a family member - at the conclusion of the transaction they instruct you
bring the form to a ticket office for in-person endorsement.

But out of curiousity: How would this work if one was in the boondocks and there was no SQ office nearby?)

Do not take any chances with this - it is just not worth it.