Question about Sennheiser HD 25-1 Ii

This should work with the ModMic: https://www.digitec.ch/6165338
The mic must then be plugged into the dedicated microphone input, it does not share the connection with the headphones.

My senheiser hd25 amperior even came with a headset cable from sennheiser itself....
Maybe ask the manufacturer instead of accessories from another manufacturer...