There is no microphone input jack on the PB278Q monitor.
The two 3.5mm jacks are for:
1) Audio-in from the PC to allow the built-in speakers to work.
2) Earphones or headphones.
Microphone must be plugged in to your PC.
If you read the specs, it does not mention a microphone jack:
https://www.asus.com/uk/Monitors/PB278Q/specifications/
There's is no way a microphone would work through a monitor in that configuration.