jsmithv's trick below didn't work for me. He said:
"1. Login to united.com and then see if the 50k offer appears
2. The offer never appeared for 50k, so instead I clicked on Apply Now for the 30k Business Explorer offer.
3. Once that application loaded and showed my pre-populated name, etc., I erase the URL, and replace it with this one
https://www.theexplorercard.com/UMPC...4M&REFERID=TEC
4. A new application will load, still populated with your information, but the application address is slightly different. This is the address I had when I applied for the 50k offer."
When I replace the 30k mi URL applications with the URL above, I get a reply stating "Bad Request."