簡單來說就是 TextBox 加上 DropDownList
然後輸入資料要及時傳給 DropDownList

///designer頁面
<script  type="text/javascript">
 function SetProd()
 {
    document.getElementById('<%=Txt_Prod.ClientID %>').value = document.getElementById('<%=Drop_Prod.ClientID %>').value;           
 }
</script>
----------------------------------------------------------------------------
<div style="position: relative;">                     
    <asp:DropDownList ID="Drop_Prod"  CssClass = "comb-textbox-list" runat="server" Style="width: 160px; margin-left: 30px" Onchange="javascript:SetProd();" OnSelectedIndexChanged="Drop_Prod_SelectedIndexChanged" AutoPostBack="True" Font-Names="微軟正黑體"></asp:DropDownList>
    <asp:TextBox ID="Txt_Prod"  CssClass = "comb-textbox" runat="server" placeholder="請先選擇客戶" Style="width: 167px; position: absolute;left: 0px; top: 0px;"> </asp:TextBox>
</div>