Hello,

You need to enable the AutoPostBack property of the DropDownList control so that an automatic post back occurs when the user changes the selected value without having to click some button. Then, you can redirect user in the SelectedIndexChanged event based on what value the user has selected:

<asp:DropDownList runat="server" ID="ddl" OnSelectedIndexChanged="OnChanged" AutoPostBack="true" ></asp:DropDownList>

protected void OnChanged(object sender, EventArgs e)
{ 
        // redirect the user if necessary
}

Anyway, you can use the URLHelper.Redirect(string Url) method to redirect the user.

Best regards,
Michal Legen