This is how I did it (but I have another receiver from FrySky instead of Spectrum but the principle remains the same):
-The principle is to train your receiver (Spectrum) so that when you turn off the radio (TR9x) this receiver will output on CH3 a PWM signal less than 975. I did that by creating a copy of my model in the T9x. On this copy, configure indeed the -125% for the throttle mix. Then place your throttle stick down (T9X will emit at that moment a ch3 less than 975) and follow the specific procedure with your spectrum receiver to tell it this is the output it must reproduce when loosing contact with the radio.
-At this stage when the transmitter will be turned off, your receiver will loose contact with it, this will trigger the receiver failsafe : that is reproduce the output you tought to the receiver with the step above, that is a CH3 throttle less than 975.
-The receiver will thus emit a PWM value on APM input ch3 less than 975; this should trigger the APM failsafe on throttle.
-Do not forget after configuring the receiver failsafe to re-select the first model profile on your radio (you can delete the model copy that you used to configure the failsafe)