Write and register functions in EXCEL 365?

[Zbigniew Jachimowicz]

Hi
How to easily write and register functions in EXCEL 365?
The text of the function I wrote:

Function Z_1 (z01, z02, z)
'
'Z_1 Function
'
     If z <= z01 Then
         Z_1 = 0
     else
     If z > z02 Then
         Z_1 = 0
     else
         Z_1 = (z - z01) / (z02 - z01)
     End If
     End If

End Function

I try to register in different ways and I can not do anything.