United In 2018 launched a promo called Mile Play, trying to get MileagePlus members to spend and fly more (they have previously had these under different names). They refresh these offers every few months and have launched a new one for April.
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 June 14, 2021.
You can try your luck registering for this offer here.
My account was not eligible for this (never has been) because I moved it 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.