How to add custom Thirdparty cpp files which are NOT in a Private folder and NOT precompiled and NOT in same folder as .h

Steps to Reproduce
Create a project. Add a new include folder, fill wiht header files. Add a second folder with cpp files.

Include gets found via:

string IncludePath= Path.Combine(ModuleDirectory, “Includes”);

PublicSystemIncludePaths.Add(IncludePath);

Linker error.

So the project structur is like:

Project/Source/MyModule/AComponentFromOutside/include and

Project/Source/MyModule/AComponentFromOutside/src both with many files and many folders in between, heavily using the struture inside as well (e.g. in include “../MyModule/MyFolder/mySecondFolder/” so structure MUST remain.)