United Airlines MileagePlus In 2018 launched a promo called Mile Play, trying to get members to spend and fly more. They refresh these offers every few months and have launched a new one for February.
The Mile Play promotion is tailored to each select member based on his/her recent activity with United. It may require buying more business class tickets or flying a certain number of trips through May 10, 2020.
You can try your luck registering for this offer here.
My account was not eligible for this (never has been) because I moved it to outside of North America a few years ago.
These Mile Plays may work for some while the requirements to earn the bonus miles can be onerous at times.