It's super hard to average near 40km/h on a mountain-bike for about 16 minutes, even on a relatively flat road, but there are things you can do to improve your time. First, you'll have to wait for the 'perfect' day, with incredibly strong winds on your back. Then, learn to cycle as aero as possible because aerodynamics play a huge role on speeds above 30km/h.

What's your bike transmission? How many teeth on the chainring and smaller cog on the back? Can you fit a 38 teeth chainring?

I checked your Strava and that segment is all about pacing. If you can try on a windy day and hold 40km/h on a comfortable cadence you'll get close. Don't forget to warm up really well too, at least 45 minutes, or else your legs won't move. If that is too much you may need so more interval training into your routine.