The BlowFlow, as the short mouthpiece, has a larger diameter than the mouthpiece inlet on the Yamaha P37D.
You could sand the Hohner mouthpiece, but I tested doing this on another adapted mouthpiece and it didn’t work as it don’t stay fixed because the Yamaha inlet has a certain angle.
You can try another way to do that and if it works, let me know.
I tried using this: LINK