Save web settings to config file

I have the following VB code that works fine, I need this ti be conveeted to C#, but it seems C' does things differently.

        If chkGoingFlying.Checked Then
            Config.AppSettings.Settings.Item("GoingFlying").Value = 1
        Else
            Config.AppSettings.Settings.Item("GoingFlying").Value = 0
        End If

        Config.Save()

This is a web application and I have a reference at the top  - Private Config As Configuration.

From searching I'm confused whether to use CongigurationManager or WebConfigurationManager. Neither works.

Is anyone able to help.

Andy