MIDI. If you have a MIDI controller or a keyboard you can just input a bass line via MIDI.

If you have a guitar you can record a single note bass line and either lower the recorded audio track one octave or use an audio to MIDI program to change it to MIDI. There are a number of programs that can do this conversion and some are free. I believe RealBand can do this if you are on a Windows system. You may have to edit the converted MIDI as there may be some mistakes, especially on the free conversion programs. Melodyne is the best program and with a monophonic line like most bass lines the least expensive can be used.

If you have non of the above you can step entry a MIDI bass track.

I hope this helps.

I think my wife has started to show the first signs of dementia.
She said she can't remember what she ever saw in me!

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware