My setup:
PX4 FW 1.8.2 stable
QGC 3.5.0
ESC with 5VBEC
QGC-PX4 Parameter:
System -> SYS_USE_IO enables/disabled both tested
Airframe Reference:
—————————
Generic Flying Wing
MAIN1: left aileron
MAIN2: right aileron
MAIN4: throttle
Connection between Pixhawk 4 --> PM board:
—————————————————————
I/O PWM OUT --> I/O PWM IN
FMU PWM OUT --> FMU PWM IN
Wiring:
———
ESC signal to M4 PMB
or other try
ESC signal to FMU-PWM-out PMB
ESC +/- to FMU-PWM-out PMB for 5V power to servos
Servo 1 signal to M1 PMB
Servo 1 +/- to FMU-PWM-out PMB
Servo 2 signal to M2 PMB
Servo 2 +/- to FMU-PWM-out PMB
I have no more ideas for solutions….
Video to explain better!
Have you pressed the safety button? Is it armed? Is RC calibrated?
Yes of course 
See the video link at end of my post.
Have you tried calibrating the ESC using QGC?
so i have new info´s. The receiver in the video ist the T-Motor T60A with default “low” timing. I use for test with another receiver the Hobbywing X-Rotor 40A, with intermediate timing (default). The throttle works now, with ECS signal to M4 on the PDB 
Now what’s the problem with the T-Motor T60A (timings low/med/high). I habe change to med and also throttle functional.
Whats the solution with timings? What is need for px4 or what ist the problem with timing?!?
too happy… 
all timings testet, beep-beep-beep after ARMING; with the Hobbywing receiver turn off the beep, with ARMING.
But what the problem with the T-Motor receiver, the 400hz? The Hobbywing has up to 621hz.
ESC calibration in QGC was fine.
THE SOLUTION … more or less …
The manual of ECS say “beep…beep” = “throttle stick is not in lowest position”. But when i connect ESC signal over M4 on the PDB no error beep. Only connect over FMU-PWM-out from PMB, the ECS beep, also with SYS_USE_IO disabled.
I’m happy… 
If you want to prevent it from beeping when disarmed you need to use one of the params to set the “disarmed” PWM value:
https://dev.px4.io/en/advanced/parameter_reference.html#PWM_AUX_DIS4 (or whatever channel), or
https://dev.px4.io/en/advanced/parameter_reference.html#PWM_MAIN_DIS4 (or whatever channel).
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Good note, spares the nerves
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