Update 1/14/22: From reader Kyle: For those experiencing issues with payment not going through, add the card to your united account as a payment method and then use the saved payment card to pay for the travel bank purchase. That work around is working at the moment.
Update 1/8/22: Travel credits now posting as well.
The United Airlines TravelBank feature was down for a few days, but has now returned.
United TravelBank link
Lots of people like using their AmEx incidental airline credit to load their United TravelBank account with funds which can be used for the next 5 years. Anecdotally, these purchases have worked to trigger the airline credit; hopefully that lasts.
Just be sure to select United as your Amex airline option first. Also be sure not to leave the TravelBank account dormant for more than 18 months. (Update: some speculate that the 18-month dormancy does not apply to the newer TravelBank loads. Regardless, if you load your TravelBank once every 18 months, or you use up some of your TravelBank balance within the 18 months, that resets your 18-month clock.)
Hat tip to FM and to reader Mike