invoke   postmessage [hwnd],wm_whatever,0,0
      cmp      eax,-1 ;return code from hwnd?
      je         negative_one
      invoke   messagebox, 0, error, error, 0
      jmp       finish
negative_one:
      invoke   messagebox,0,ok,ok,0
finish:
      invoke    exitprocess,0
    

wm_whatever:
       ...
       ...
       mov    eax,-1
finish:
       pop edi esi ebx
       return
    

The PostMessage function places (posts) a message in the message queue associated with the thread that created the specified window and then returns without waiting for the thread to process the message.

format PE GUI 4.0
entry start

include '%fasminc%\win32a.inc'

section '.data' data readable writeable

  _title db 'Win32 program template',0
  _class db 'FASMWIN32',0

  mainhwnd dd ?
  hinstance dd ?

  msg MSG
  wc WNDCLASS

section '.code' code readable executable

  start:

        invoke  GetModuleHandle,0
        mov     [hinstance],eax
        invoke  LoadIcon,0,IDI_APPLICATION
        mov     [wc.hIcon],eax
        invoke  LoadCursor,0,IDC_ARROW
        mov     [wc.hCursor],eax
        mov     [wc.style],0
        mov     [wc.lpfnWndProc],WindowProc
        mov     [wc.cbClsExtra],0
        mov     [wc.cbWndExtra],0
        mov     eax,[hinstance]
        mov     [wc.hInstance],eax
        mov     [wc.hbrBackground],COLOR_BTNFACE+1
        mov     [wc.lpszMenuName],0
        mov     [wc.lpszClassName],_class
        invoke  RegisterClass,wc

        invoke  CreateWindowEx,0,_class,_title,WS_VISIBLE+WS_DLGFRAME+WS_SYSMENU,128,128,192,192,NULL,NULL,[hinstance],NULL
        mov     [mainhwnd],eax

        invoke  ShowOK,eax

  msg_loop:
        invoke  GetMessage,msg,NULL,0,0
        or      eax,eax
        jz      end_loop
        invoke  TranslateMessage,msg
        invoke  DispatchMessage,msg
        jmp     msg_loop

  end_loop:
        invoke  ExitProcess,[msg.wParam]

proc WindowProc, hwnd,wmsg,wparam,lparam
        push    ebx esi edi
        cmp     [wmsg],WM_DESTROY
        je      wmdestroy
        cmp     [wmsg],WM_USER
        je      wmuser
  defwndproc:
        invoke  DefWindowProc,[hwnd],[wmsg],[wparam],[lparam]
        jmp     finish
  wmuser:
        or      eax,-1
        jmp     finish
  wmdestroy:
        invoke  PostQuitMessage,0
        xor     eax,eax
  finish:
        pop     edi esi ebx
        return
endp

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL',\
          errormsg,'ERRORMSG.DLL'

  import kernel,\
         GetModuleHandle,'GetModuleHandleA',\
         ExitProcess,'ExitProcess'

  import user,\
         RegisterClass,'RegisterClassA',\
         CreateWindowEx,'CreateWindowExA',\
         DefWindowProc,'DefWindowProcA',\
         GetMessage,'GetMessageA',\
         TranslateMessage,'TranslateMessage',\
         DispatchMessage,'DispatchMessageA',\
         LoadCursor,'LoadCursorA',\
         LoadIcon,'LoadIconA',\
         PostQuitMessage,'PostQuitMessage'

  import errormsg,\
         ShowOK,'ShowOK'    

format PE GUI 4.0 DLL
entry DllEntryPoint

include '%fasminc%\win32a.inc'

section '.code' code readable executable

proc DllEntryPoint, hinstDLL,fdwReason,lpvReserved
        mov     eax,TRUE
        return
endp

ok db 'ok!',0

proc ShowOK, hWnd
        invoke  SendMessage,[hWnd],WM_USER,0,0
        cmp     eax,-1
        jne     @f
        invoke  MessageBox,[hWnd],ok,0,0
    @@:
        return
endp

section '.idata' import data readable writeable

  library user,'USER32.DLL'

  import user,\
         MessageBox,'MessageBoxA',\
         SendMessage,'SendMessageA'

section '.edata' export data readable

  export 'ERRORMSG.DLL',\
         ShowOK,'ShowOK'

section '.reloc' fixups data discardable    

The return value specifies the result of the message processing and depends on the message sent.