I, too, use a Corsair Harpoon. I wasn't even aware the iCUE software had this functionality; I usually only use it to turn the lights off (by telling them to be black) and change the "sniper button" to a simple toggle between "normal" and "high-precision", because more precision levels than that just causes my mouse to feel stupid when my cats walk across my desk.

Today I learned.

I was able to make a functioning dodge-roll macro and assign it to my "back" button as follows:

• Open the iCUE software
  
• Edit the profile
  
• Select "Key Assignments"
  
• Click the "+" button in the box on the bottom-left.
  
• Select "Macro" in the second box ("Assignment Type")
  
• Click on the box inside the third box ("Key"), then press the thumb button to assign the macro to that button.
  
• In the final box ("Macro"), I pressed the "record" button, then pressed LCtrl, then Space, then released Space, released LCtrl, then pressed and released LCtrl one final time (to toggle "crouch" back off)... followed by clicking the record button again to stop the recording.
  
• I then adjusted the delay timings by clicking on the "XXX ms" boxes in the macro window until the macro looked something like this:
  1. LCtrl Press
     
  2. Delay 5ms
     
  3. Space Press
     
  4. Delay 30ms
     
  5. Space Release
     
  6. Delay 5ms
     
  7. LCtrl Release
     
  8. Delay 30ms
     
  9. LCtrl Press
     
  10. Delay 20ms
      
  11. LCtrl Release

Testing in-game confirmed this caused me to dodge-roll forward with no other keys pressed, or dodge-roll in the appropriate direction if pressed in combination with a movement key.

The timings aren't perfect; if I press the button again too soon after using it, I come out of the roll into a crouch... but pressing it again seems to fix it, so I'll call this "a good start" on a functional macro.

As an aside, I suspect the "Delay 30ms" after the second "LCtrl Press" could be shorter, and possibly the "Delay 20ms" between the final press and release of LCtrl as well... but it makes me feel icky to use a macro (no judgement on you, it's totally my hangup), so I'll leave the fine-tuning in your hands.