Do you mean audio from Reaper? If so, there are a couple of Youtube videos explaining how. It's not trivial (or at least it wasn't for me). See, e.g.:
https://www.youtube.com/watch?v=hRWIJqgRUXc
https://www.youtube.com/watch?v=rxZ3o1aTyPY&t=704s
Basically, you need to send whatever audio you want to generate to a track in Reaper, and add a hardware output on that track using ReaRoute to get the audio from Reaper to Jamulus, then set Jamulus to use ReaRoute channels as its ASIO inputs and outputs, then tell REAPER to use Rearoute as an input on another track, to monitor what's comong back from Jamulus. That's oversimplifying, the first video shows it pretty clearly. Both videos also explain how to get Reaper audio into OBS streaming software to send to Facebook Live, or Youtube, or Zoom, etc. etc.
I've done the Reaper to Jamulus thing, and it's pretty fun to be able to use my MIC, virtual instruments with a MIDI keyboard, and a guitar using VSTs for effects, in a Jamulus jam.
$0.02
Dave_t