To do this, you need to find the owner and conclude a contract of sale with him. However, in some cases, you can do it differently. If the power of attorney provides for the possibility of transferring the rights to dispose of the car to another person, then such a document must be issued to a friend, and then buy a car from him. But buying a car from yourself will not work. According to the Civil Code https://hide.vin/products, a transaction must have two parties.