If you are a M+ member and receive their email notices/advertisements, you may receive an offer. The offers vary by previous purchases, number of flights on United, using miles for flights. I am not really sure of the algorithm of who receives offers. My wife did not receive the offer and she flies with me. I use my M+ Visa card more than she does, so that may be the link.
If you are new to all of this, it will take a while to understand the process. Keep reading people's experiences in this web page.
Good travels,