Simply make a variable that is $true if you are waiting for the key to be pressed.

Then when the key is pressed, if variable exists, do what youw ant.

Code:
alias key_ {
  set %key_ $true
  echo -a Ok Im waiting for you to press the key.
}

alias f5 {
  if (%key_) {
    echo -a I waited for the key to be pressed and you just pressed it.
    unset %key_
  }
}



Type /key_ to start this. Press F5 after.