To get the Minecraft anniversary creeper cape on Java Edition, players needs to visit official 15th anniversary page on Minecraft website and scroll down to find the option to claim creeper cape ...

Unfortunately, for Minecraft Java Edition, there are no skins that will automatically incorporate a cape. However, in Bedrock Edition, there are some skin packs that come with capes included.

The only official ways to get a cape in Minecraft are by attending special events or buying skin packs, but Java Edition players can also use mods.