You cannot add an exclamation mark to your Fortnite display name through Epic Games’ official name-change system
Fortnite names only allow certain characters, and exclamation marks are not supported
If you want a name with an exclamation mark, you would need to change it on a linked platform account only if that platform allows it, but Fortnite may still not display it
Use a supported special character or a different name style instead
Check Epic Games’ current display name rules before changing your name