If a ticket is canceled within 24 hours of purchase by United Airlines, full refunds will be issued, provided that the reservation was made at least 7 days in advance of the departure date. This holds for all reservation kinds, even tickets for Basic Economy.

To make and manage reservations using the website or mobile application, consult United Airlines Reservations. If you cancel your ticket within the 24-hour window, as per United's cancellation policy, your refund will be issued back to the original payment method.
https://uspto.data.commerce.gov/nominate/528283