Literally just last night, I ordered a PoE injector that was recommended from another community member here who said it works out of the box, no faffing around with any power, ports or additional dongles.
They recommended the "Ubiquiti Networks Instant 802.3af Indoor Gigabit PoE Converter" (model INS-3AF-I-G, $25 at Amazon US).
I'll know in a couple of days if this works, and can report back. I needed a clean, inline solution, and this could be it for me (and possibly yours as well!)
The key is to make sure you get the 24V version, not the 48V version. Unifi has a few models, indoor, outdoor, 24V, 48V, and PoE to USB. Choose wisely. The one I linked above, is the 24V, Indoor, PoE Injector version.