something like below may helps you.
<asp:DropDownList ID="ddlValues" OnSelectedIndexChanged="ddlValues_OnSelectedIndexChanged"
runat="server">
<asp:ListItem Text="Apple" Value="1"></asp:ListItem>
<asp:ListItem Text="Microsoft" Value="2"></asp:ListItem>
</asp:DropDownList>
protected void ddlValues_OnSelectedIndexChanged(object sender, EventArgs e)
{
Response.Redirect("YouURL.aspx?Value=" + ddlValues.SelectedValue);
}