RDR2 is now released, so it's impossible for spoilers to be completely contained, especially when it comes to gameplay features. Please be mindful of others and use Spoiler Tags especially when talking about the story, or, even more importantly, the end-game.
On PC or larger screen tablets, you can use the Eye icon on the editor:
If for some reason you cannot see this button (like on smaller mobile devices, this should be fixed soon), you can use the classic BBCode tags, as follows:
[spoiler]Your Text Here[/spoiler]
If you are creating a thread, please add the Spoilers tag both as a tag and as a prefix, so it can be neatly indicated as a spoiler thread:
Thanks for your understanding, be mindful of those who do not yet have the game.