Decrescendo with specific dynamics

select the sffz and assign a dynamic value in inspector. then use the hairpin btween the sffz and the ff

Note that sffz is a "special" dynamic, which is why you need to set this up manually. Normal dynmamics are designed to affect all notes from that point forward. sffz affects only the note is placed n. So technically, a decrescendo after an sffz doesn't really make sense - the dynamic is *already* assumed to immediately go back to whatever it was before. But if you wish to make this more explicit for some reason, then indeed, you could set it up manually like this.