Can you use earphones as microphone?

The combined headphone/boom mic, usually have two 3.5mm plugs, color coded green and pink.

Plug the green one into the headphone or speaker output and the pink one into the mic socket.

If the headset has a single 3 ring,(stereo) 3.5mm plug, then it is generally not compatible with computer use.

You may be able to find an adapter, that converts a stereo plug to two separate plugs.