Treasure Hunt's are missions/side-missions within Far Cry 6 that you'll have the chance to take part in during the game, and they're going to be great for obtaining additional loot and XP for your journey.
The High Supply mission is going to be located in Lozania, and you should see it on your map via a diamond logo.
Once you've begun the mission, all you really have to do is to follow the ledges marked by orange paint. As, it'll take you atop a mountain the more you climb, and before you know it, you'll have the treasure.
However, there are two areas that are confusing players. The first is located in a cave, where you must cross a long corridor and make a jump. It's too long to jump across, so instead you'll need to climb on the colored rocks on either side of the corridor (you will need to jump a few times, so keep looking out on either side).
The next is a piece of orange netting holding up some rocks. If you've completed the El Este mission, you'll know what to do. Shoot the open spot in the middle of the netting a few times and it will release the rocks allowing you to proceed.