Source Code :


<form id="form1" runat="server">
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:DropDownExtender ID="TextBox1_DropDownExtender" runat="server"
DynamicServicePath="" Enabled="True" DropDownControlID="Panel1" TargetControlID="TextBox1"
HighlightBackColor="Orange"
DropArrowBackColor="OrangeRed">
</asp:DropDownExtender>
<asp:Label ID="lbl1" runat="server"></asp:Label>
<asp:Panel
ID="Panel1"
runat="server"
BorderColor="Pink"
BorderWidth="2"
>
<asp:RadioButtonList
ID="RadioButtonList1"
runat="server"
OnSelectedIndexChanged="RadioButtonList1_SelectedIndexChanged"
AutoPostBack="true"
>
<asp:ListItem>Crimson</asp:ListItem>
<asp:ListItem>Orange</asp:ListItem>
<asp:ListItem>HotPink</asp:ListItem>
<asp:ListItem>DarkRed</asp:ListItem>
</asp:RadioButtonList>
</asp:Panel>


</form>



Code Behind:


protected void RadioButtonList1_SelectedIndexChanged(object sender, EventArgs e)
{
TextBox1.Text = RadioButtonList1.SelectedItem.Text;
}



In the above code on clicking on textbox you will get radio list after selecting the value the selected value would be displayed in the textbox