Add Message Effects
It’s easy to miss the arrival of effects in the Big Sur Messages app, since there’s no “Send” button to bring them up by long-pressing. Instead, you simply click on the “Apps” button to the left of the text field, and you’ll see “Message Effects” as one of the options.
Just like on the iPhone and iPad, you’ll need to make sure you’ve entered some text before you can actually access the effects. This is a bit less obvious on the Mac than it is on iOS, since in that case, the “Send” button doesn’t appear until you’ve actually typed something into the message field.