The A8S has pinouts for PPM or IBUS, so just make sure you are using the correct one (IBUS) and that it is connected to the SBUS pad on the omnibus. The power and ground wires in your diagram are correct.
Once that is confirmed, the next thing would be to make sure you have bound your receiver to your radio correctly (this terrible video may help 
)
Then the last thing is to setup betaflight (serialRX on UART1, and set the receiver type to be IBUS) and you should be good to go. If IBUS is not working then try SBUS as I cant recall which one you actually need to use (and it also depends on the settings on your flysky I beleive)… I cant remember exactly as haven’t flown a flysky radio for a good 6 months now…