This page of the guide to Far Cry 6 contains complete information on how to unlock the hidden ending of the main storyline. You will learn when you can see the hidden ending, what are its requirements, and of course, what does it look like.
For obvious reasons, the page contains major spoilers about one of the possible endings of the game. We still recommend reading the walkthrough, because the hidden ending can be easily missed.
The hidden ending in Far Cry 6 is not viewed at the very beginning of the game. Instead, you have to spend several hours with the game doing the first few main missions (operations) in the "starting" location Isla Santuario.
The last introductory quest is called Libertad Rises, during which you will successfully sabotage two enemy ships and leave Isla Santuatio with Clara. The characters will sail to the rebel base - Libertad Island.
The next operation - Guerrilla - will begin, and it's a sort of introduction to the main part of the campaign.
At the very beginning of this mission you will talk to Clara, and learn that the the Guerilla have prepared a boat for the protagonist if they really don't want to help and want to leave Yara.
After watching the cut-scene, don't do the Guerrilla quest, but instead enter the boat.
You don't have to sail in any set direction. Sail away from Libertad Island and wait for the secret ending to automatically start.
Another cutscene will reveal that the protagonist has successfully escaped from Yara and that they are now relaxing on a tropical beach. The radio broadcast will tell you about the tragic events that occurred on the island, followed by the ending credits.
Important note - Unlocking the hidden ending will not reset your progress and you will not have to restart the entire campaign. After you return to the main menu, you can choose to continue the game. You will be brought back to the very beginning of the Guerrilla quest, so right before the moment you choose the hidden ending. This time you can ignore it and get on with the Guerrilla mission.