How to access to App Constant values from code [CustomFunction]

Hi there!

i'm trying to get/use the Strings and Integers from my Constant values on a CustomFunction, but seems impossible.

I've tried with:

return FFAppConstants.ServicesCategoryID;

where 'ServicesCategoryID' is one of my Constants variable name.

The editor is showing continously an error under 'FFAppConstants' :

Undefined name 'FFAppConstants'.

I've also tried to apply the 4 'Quick Fix' solutions, basically 4 imports of different packages. But no one seems to solve the problem.

Any advice?

Thank you.