<script language="javascript">
function addString(str)
{   
  obj=document.getElementById("txt");   
  obj.focus();   
  if(document.selection==null)   
  {   
      var iStart = obj.selectionStart;   
      var iEnd = obj.selectionEnd;   
      obj.value = obj.value.substring(0,iEnd) + str + obj.value.substring(iEnd, obj.value.length);   
  }
  else   
  {   
      var range =  document.selection.createRange();   
      range.text += str;   
  }     
}
</script>
<asp:TextBox ID="txt" runat="server" Text="oooooo"></asp:TextBox>
<input type="button" value="+" onClick="addString('k')">