If you set OUTPUT TO to “USB” in the Digitakt’s SETTINGS > MIDI CONFIG > PORT CONFIG menu, then the Digitakt can send MIDI messages to Bitwig.
Because Digitakt is not a USB host, you probably need a USB host device like a computer between the DT and your USB keyboard. Which USB keyboard do you have?
I tried to set this already in the Digitakt’s settings. And tried to send midi notes to, let’s say channel 1. I use the VSTI Omnisphere and configured something for channel 1, but nothing came out of my DAW nor Digitakt.