Is it permissible to benefit from a shopping voucher I won in a competition whose answer was not dependent on watching its associated video, and what should I do with it if it is not permissible?

There is no objection to utilizing the purchase voucher to participate in the competition and answer the question, especially since the stores' purpose is to advertise and market their goods on social media platforms. This is achieved simply by accessing and browsing their accounts, and sending a tag or notification to a friend is considered real advertising for the store and an invitation to join the competition.