try like this
create a drop down in aspx page and create a SelectedIndexChanged
event for that dropdownlist then assign dropdownlist value to textbox
like
Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
TextBox1.Text = DropDownList1.SelectedItem.Text
End Sub
and set autopostback= true for dropdownlist
like
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
<asp:ListItem>12</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
<asp:ListItem>5</asp:ListItem>
</asp:DropDownList>