Send a EM_SETTABSTOPS message to the Memo control. The following example sets the first 5 tab stops to 20 pixels. Example:

procedure TForm .FormCreate(Sender: TObject);
var
  DialogUnitsX : LongInt;
  PixelsX : LongInt;
  i : integer;
  TabArray : array[0..4] of integer;
begin
  Memo .WantTabs := true;
  DialogUnitsX := LoWord(GetDialogBaseUnits);
  PixelsX := 20;
  for i :=   to 5 do begin
   TabArray[i -  ] :=
     ((PixelsX * i ) * 4) div DialogUnitsX;
  end;
  SendMessage(Memo .Handle,
              EM_SETTABSTOPS,
              5,
              LongInt(@TabArray));
  Memo .Refresh;
end;