The key has been found! Thanks to a user on Twitter for finding the key. I have created a video showing where the key is and how to get it. Tips:
- Look at the map in the upper right corner to see my location. Guide:
1. From the Trials Fusion menu, click "Create" and you'll be put into the trials fusion world.
2. Go to the location shown in the video.
3. Put down the start and finish markers for the track. (Anywhere)
4. Add a new object > Environment > Terrain Shaping.
5. Go to object properties > Select "Holes" for category > Select "Crater 5" for decal.
6. Copy and paste that crater on the mountain until you find the key as shown in the video.
7. Pick up the key... the achievement should pop shortly.
The guy who found the key: https://twitter.com/nathan5661