On linux I like to make software midi devices and I usually want bitwig to see that as input but it doesn't, here's a session where I was able to get it to work:
cjm@cjm-desktop:~/projects/anchor_system_organization/c-keyboard$ sudo modprobe snd_virmidi midi_devs=1
cjm@cjm-desktop:~/projects/anchor_system_organization/c-keyboard$ ./build/c-keyboard
APIs
API #0: unspecified / default
API #2: ALSA
API #3: Jack
Choose an API number: 2
Would you like to open a virtual output port? [y/N] n
Output port #0: Midi Through:Midi Through Port-0 14:0
Output port #1: Virtual Raw MIDI 3-0:VirMIDI 3-0 28:0
Choose a port number: 1