When I had the Chaser Rewards Plus (which for me Chase discontinued over a year ago), the way I had to redeem for UA miles was to go to the redemption center for gift cards, etc, and search "United". Then the certificates for 5000 miles for 6000 points (or something like that) would come up, but not as the very first search return! So if it still exists, that's presumably how you use it.

But since I haven't had access to Chase Rewards Plus for over a year now, I can' tell you whether that still works or not.

(Back then, the certificate was the only way to transfer. You'd redeem points for the certificate, which they'd mail to you, at no cost, within a week or so. They'd you'd go online to a website mentioned on the certificate, enter your UA number, enter the certriicate codes, and it would deposit the miles into your UA account a few days later.)