<div style="z-index: 0; visibility: visible; clip: rect(0px 105px 80px 85px); position: absolute">
  <asp:DropDownList ID="ddlModel" runat="server" Style="z-index: -1" Width="105px"
    onchange="getModelTo(this)">
    <asp:ListItem Value="1">SSM-001</asp:ListItem>
    <asp:ListItem Value="2">DDW-523</asp:ListItem>
    <asp:ListItem Value="3">QSD-009</asp:ListItem>
  </asp:DropDownList>
</div>
<asp:TextBox ID="txtModel" runat="server" Style="z-index: 1px; position: absolute"
  Font-Size="9pt" Width="95px" Height="16px" MaxLength="50"></asp:TextBox>
<script type="text/javascript">
  function getModelTo(e) {
    document.getElementById("txtModel").value = e.options[e.selectedIndex].innerText;
    document.getElementById("txtModel").select();
  }
</script>

<div style="z-index: 0; visibility: visible; clip: rect(0px 105px 80px 85px); position: absolute">
  <asp:DropDownList ID="ddlModel" runat="server" Style="z-index: -1" Width="105px"
    onchange="getModelTo(this)">
    <asp:ListItem Value="1">SSM-001</asp:ListItem>
    <asp:ListItem Value="2">DDW-523</asp:ListItem>
    <asp:ListItem Value="3">QSD-009</asp:ListItem>
  </asp:DropDownList>
</div>
<asp:TextBox ID="txtModel" runat="server" Style="z-index: 1px; position: absolute"
  Font-Size="9pt" Width="95px" Height="16px" MaxLength="50"></asp:TextBox>
<script type="text/javascript">
  function getModelTo(e) {
    $("#txtModel").val($("#ddlModel").find("option:selected").text());
    $("#txtModel").select();
  }
</script>