What are replacement brake pads for the Heybike Mars (purchased this month).
I think that is a question for the seller of the bike you have. It would impossible to help you without at least knowing the brake Mfg and model #.
Can you supply that information if you don't get information from the seller? Maybe remove the current brake pads and take some detailed pictures of the pads. That way maybe someone can find a replacement pad. Fortunately most of these brakes use similar pads.
I visited the HeyBike website they don't show specific information on the brakes on your bike. But from the photos of your model on their page is looks like you have mechanical/single piston brakes of unknown mfg.