How to set Custom Text into DropdownList in ASP.Net

<asp:DropDownList AppendDataBoundItems="true" ID="yourListId" runat="server">
    <asp:ListItem Text="Select something" Value="-1" />
</asp:DropDownList>

function addFirstItem(list, text, value)
{
    var newOption = document.createElement("option");
    newOption.text = text;
    newOption.value = value;
    list.options.add(newOption);
}

addFirstItem(document.getElementById("yourListId"), "Select something", "-1");

$("#yourListId option:first").before("<option value='-1'>Select something</option>");

<asp:DropDownList AppendDataBoundItems="true" ID="yourListId" runat="server">
  <items>
    <asp:ListItem Text="Select something" Value="-1">--Select Something--</asp:ListItem>
  </items>
</asp:DropDownList>