It’s easier to credit using a narration bubble:
NARR
I’d like to credit @_________, @_________, @__________ for featuring in this story!
Or something along the lines of that. 
You could have it as a speech bubble too that your character says, if you want:
YOURCHAR (animation)
I’d like to credit @_________, @_________, @__________ for featuring in this story!
But you cannot have the @ symbol at the beginning of the dialogue/narration nor at the beginning of a new line because it’ll think you’re trying to give it a command.