You should not need to add this path! Please make sure you use the default AurduinoMulticoreUser project as a basis for your own work. This has all the paths and settings already configured so that all the Arduino-style libraries and functions are available.
Assuming you are using the free PLS UDE debugger, there is nothing special you need to do. Your C file should be in the sketches directory and the debugger will find it, provided at least one function in your file is called from setup(), Loop() etc.