A MIDI Thru port is a hardwired mirror of the Input Port.
PreSonus does not have MIDI Thru ports on any of our devices, nor will you find this on almost any audio interface. Thru ports are prevalent on MIDI Patchbays and on older hardware synthesizers.
You'll need to install a program to reroute MIDI Input to the Output port as a Thru port.
For Mac there's this freebie - https://apps.apple.com/us/app/midi-thru/id873634211?mt=12
For Windows - MIDI Yoke may work, it's pretty old though, a Google search will find other options like LoopBe1.
We haven't tested any of these, you'll have to do some trial and error to find one that works for you.