In the first week of Fortnite Season 5 you will have to find a treasure in “Fortnite Battle Royale” (Follow the Treasure Map found in Risky Reels).
Like other weekly treasure challenges, you can go straight to the treasure without having to find the treasure map and pick it up.
At the point shown in the video, you can find the golden star symbol. Then finish the game (or get killed) to properly save the challenge. If you leave the game manually without dying, it may not update the challenge. You can see everything under the Battle Pass challenges in the lobby. You need a Battle Pass for 1000 V-Bucks to unlock weekly challenges!
An overview of all challenges of the first week of Season 5 can be found here.