procedure TMainForm.WMCopyData(var Msg: TWMCopyData);
var S:String;
    AData:TCopyDataStruct;
begin
  case Msg.CopyDataStruct.dwData of
    WM_MYCOMMAND: begin
                                    Msg.Result:=1;
                                    S:=String(PChar(Msg.CopyDataStruct.lpData));
                                    // в s - "сообщение"
                                 end;
   end;
end;

var WTargetWindow:HWND;

function EnumWindowsProc(WHandle: HWND; lParam: LPARAM): BOOL; export; stdcall;
var WindowName:array[0..MAX_PATH] of char;
    AClassName:array[0..1024] of char;
    S1:string;
begin
  Result:=True;
  GetWindowText(WHandle,WindowName,MAX_PATH);
  S1:=String(WindowName);
  GetClassName(WHandle,AClassName,1024);
  if String(AClassName)='TMyTargetMainForm') then
     begin
      WTargetWindow:=WHandle;   // find target form
      Result:=False;       
     end;
end;

function FindTargetWindow:HWND;
begin
  WFadWindow:=0;
  EnumWindows(@EnumWindowsProc,LPARAM(PChar('TApplication_Class_Name')));
  Result:=WFadWindow;
end;

procedure SendDataStrToTarget(ACommand,Flags:integer; S:String);
var TargetHWND:HWND;
    AData:TCopyDataStruct;
begin
 TagretHWND:=FindTargetWindow;
 if TargetHWND<>0 then
    begin
      AData.dwData:=ACommand;
      if Trim(S)='' then
         begin
           AData.cbData:=0;
           AData.lpData:=nil;
         end
      else begin
             AData.cbData:=StrLen(PChar(S))+1;
             AData.lpData:=PChar(S+#0);
           end;
      SendMessage(TargetHWND,WM_COPYDATA,wParam(Flags),lParam(@AData));
    end;
end;

// использование
SendDataStrToTarget(WM_MYCOMMAND,'My message to target');

function FindTargetWindow:HWND;
begin
  WTargetWindow:=0;
  EnumWindows(@EnumWindowsProc,LPARAM(PChar('TApplication_Class_Name')));
  Result:=WTargetWindow;
end;

procedure SendDataStrToTarget(ACommand,Flags:integer; S:String);

// -----------
      else begin
               AData.cbData:=Length(S)*SizeOf(Char)+1*SizeOf(Char);
               AData.lpData:=PChar(S+#0);
           end;
// -----------