Bottom right of Vehicle page you set an unused Accessory# (such as "Accessory0") to the Output channel number you want (it sounds like you want 6 in your case).
You then go to Input page and tell it that Accessory# (Accessory0) comes from the same Type, Number, Min, Max as the Throttle is using. I don't think that Neutral matters as long as it is between Min and Max.
You then go to Output page and configure a possibly very different output for Accessory0 than the input is. First set Min=1000 and Max=1600. In your case it sounds like you need reversing here (check mark the Reversed box for Accessory0), so that buzzer comes on when it is BELOW 1500. You now have Reversed=checked, Min=1600, Max=1000.
This should make it so that when throttle stick is below 1/16th the buzzer comes on.
You can power everything up with it connected to USB / GCS, go to Output page and watch what the transmitter throttle stick now does to the Accessory0 outputting on channel 6.
Careful of spinning props during testing!!!
Might need to be armed or use the "Test Outputs" to get the Acessory0 Output to move. I recall that "Accessory"s move even when not armed, so try
without arming/testoutputs first.
For extra credit you can make it come on only when the throttle stick and trim are both low. You would configure the Accessory0 Input page min to extend lower by seeing what the number needs to be when both the transmitter stick and trim are low. Then tweak the Accesssory 0 Output page to move the Min=1600 to something like Min=1520. Find the actual number where the buzzer comes on and make low throttle (neutral trim) be on one side of that number and low throttle low trim on the other side of that number with the number
half way between low-throttle-neutral-trim and low-throttle-low-trim.
I can just imagine the smile on your face when you get it configured to only beep at low throttle with low trim. Just like using the trim to give you a separate channel that you don't have.
Please post back to let me know.