How to create a New system function

[NYExcel]

How can I setup an function which I can view and use when I click “insert
function” or as a system?
e.g.
Public Function NewNumber(IDRange As Range)
NewNumber = IDRange.Cells(1, 1).Value + 1
End Function
I want the function NewNumber can be as part the list functions when click
“insert function” (“fx”)