Search here for Luhn:
Do you have FileMaker Pro Advanced? If so, you can use any of the custom functions there. If not, you'll have to have someone add the CF to your file.
However, you don't have to implement it as a custom function, as long as it's not recursive. For example, this one could be implemented as a standard calculation:
Thank you very much indeed! Yes I use FMPA14.0 for Mac using El Capitan.