procedure SendShift(H: HWnd; Down: Boolean);
var vKey, ScanCode, wParam: Word;
    lParam: longint;
begin
  vKey:= $10;
  ScanCode:= MapVirtualKey(vKey, 0);
  wParam:= vKey or ScanCode shl 8;
  lParam:= longint(ScanCode) shl 16 or 1;
  if not(Down) then lParam:= lParam or $C0000000;
  SendMessage(H, WM_KEYDOWN, vKey, lParam);
end;

procedure SendCtrl(H: HWnd; Down: Boolean);
var vKey, ScanCode, wParam: Word;
    lParam: longint;
begin
  vKey:= $11;
  ScanCode:= MapVirtualKey(vKey, 0);
  wParam:= vKey or ScanCode shl 8;
  lParam:= longint(ScanCode) shl 16 or 1;
  if not(Down) then lParam:= lParam or $C0000000;
  SendMessage(H, WM_KEYDOWN, vKey, lParam);
end;


procedure SendKey(H: Hwnd; Key: char);
var vKey, ScanCode, wParam: Word;
  lParam, ConvKey: longint;
  Shift, Ctrl: boolean;
begin
    ConvKey:= OemKeyScan(ord(Key));
    Shift:= (ConvKey and $00020000) <> 0;
    Ctrl:= (ConvKey and $00040000) <> 0;
    ScanCode:= ConvKey and $000000FF or $FF00;
    vKey:= ord(Key);
    wParam:= vKey;
    lParam:= longint(ScanCode) shl 16 or 1;
    if Shift then SendShift(H, true);
    if Ctrl then SendCtrl(H, true);
    SendMessage(H, WM_KEYDOWN, vKey, lParam);
    SendMessage(H, WM_CHAR, vKey, lParam);
    lParam:= lParam or $C0000000;
    SendMessage(H, WM_KEYUP, vKey, lParam);
    if Shift then SendShift(H, false);
    if Ctrl then SendCtrl(H, false);
end;

...

В коде 

procedure SendingString(wnd_target: HWND; s: string);
var
  i: integer;
begin
  for i:=1 to length(s) do SendKey(wnd_target, s[i]);
end;