What the correct way to write Excel UDF's so it work on Desktop, Web and Mobile Office?

With the introduction of Excel on Android and iOS, I'm not sure what the correct method of creating a UDF (User Defined Function) that will work across all versions of the Office 365?

I want my UDF to work on iPad, Android as well as Desktop. What's the official Microsoft way of supporting all of these versions of Office for UDF's?