How to add prompts to user defined function?

I have added the Count_Chars user defined function to my spreadsheets (see below).
I would like to have prompt or help information provided as in in the standard IF function:


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Function Count_Chars(Target As String, rng As Range)
   Count = 0
   If Target = "" Then GoTo Done
    For Each c In rng
      N = InStr(1, c.Value, Target)
      While N <> 0
       Count = Count + 1
       N = InStr(N + 1, c.Value, Target)
      Wend
    Next c
  Count_Chars = Count
 Done:
 End Function