1. Visit UnitedExplorerCard.com/50bonus
2. On the page, log in into your United MP account and if your page is redirected to https://www.theexplorercard.com/50kAFW then you qualify for the 55K offer.
You will need to spend $2,000 within 3 months to get 50,000 miles. And if you add an authorized user and s/he uses the card for the first time, you will receive an additional 5,000 miles. Like always, the $95 annual fee is waived for the first year.