That is not possible. Windows services cannot run executable applications or invoke them, unlike Windows applications like Winforms, Console or WPF. Your code will work in any application other than a Windows service.
Windows has a security mechanism that causes services to run in sessions isolated from the user’s context, so there is no context where to start the executable.
Windows executables are only started in a context, be it administrator, system, or any user. Since a service is not in a specific context, but rather an isolated session, there is no place to run the application.
There is a way unsafe and not recommended to execute an executable from a service: by injecting the executable into an existing process. Therefore, this can make the system unstable and will become considered malware ever since. More details in this answer stack overflow.
I don’t understand what you mean by internal. You want your windows service to run/open another program or for it to interact with it?
– Leandro Angelo