Hello:
If you disassemble the code of your a.exe, you could see imports (with wdasm32, for instance) and there all the apis used by the application. Then, select in the import window "ExitProcess" and display it, then you are getting a "call [<address of ExitProcess>]".
You could write down in your new code a call like that you have got, or a jmp to the RVA of that call.
Cheers
Nacho_dj