WINDOWS how to obtain the active form the process ID? ? ?

It is not clear from your question exactly what you are trying to do, but I will take a stab at it.
If you are the one starting the hidden form you can get its process ID.
For example if you start a program from your code called "MyProgram" using the Process.Start method it returns a variable of type Process from which you can get the ID.

Process myProc = Process.Start("C:\\MyProgram.exe");
int processId = myProc .Id;