How to run code generation with Matlab Coder App, if C-files need to be included?

In the help of coder.ceval the code generation for the callfoo function is done on command line. There you can pass the foo.c and foo.h directly. But if you use the Matlab Coder App, then there is no place to pass the C-Files to the App. Therefore, I get a failure message which tells me that 'foo' is unknown.