In WSJT-X on the Settings dialog, Radio tab, you need to have:
Rig: Yaesu FT-817 (I use DXLab Commander for CAT control and in my case, "Rig" is set to DXLab Commander")
CAT control - Serial Port should be the COM port your uBitx is connected to
Baud rate 38400, Data Bits 8, Stop Bits Two, Handshake None
PTT Method: CAT
Split Operation: you may need to select "Fake It," I couldn't get "Rig" working when I connected directly to the uBitx
Click Test CAT. Does it turn green after a short wait? If not, something is not connecting properly and you won't get proper results until you figure that out.
If you get green, and have a dummy load or proper SWR antenna connected, click Test PTT. The uBitx should go into TX mode.
If all is well, click OK.
At this point, the frequency shown in WSJT-X should match, and follow, the frequency of your uBitx. If you choose a band / frequency from WSJT-X's drop-down, the uBitx should change to that band and frequency.
Click Tune and the uBitx should transmit power that you could use to adjust your antenna tuner. Don't tune into a high SWR or open antenna port or you may blow out the finals.
If all this is good, go make some Q's!