how to read a variable into the app.config xml file

Question

I have the following log path I want to use in a class called FilePaths.cs:

public static String LOG_PATH = AppDomain.CurrentDomain.BaseDirectory + "Log Directory";

In my app.config class, I have my log4net configureation settings. How do get the variable LOG_PATH to load into the value for 'file value'? Right now I have it hard coded to a random spot on my hard drive.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>

  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,Log4net"/>
  </configSections>

  <log4net>
    <appender name="TestAppender" type="log4net.Appender.RollingFileAppender" >
      <file value="C:\log\bla\MyTestAppender.log" />
      <encoding value="utf-8" />
      <appendToFile value="true" />
      <rollingStyle value="Date" />
      <rollingStyle value="Size" />
      <maxSizeRollBackups value="5" />
      <maximumFileSize value="2MB" />
      <staticLogFileName value="true" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%date [%thread] %-5level %logger - %message%newline"></conversionPattern>
      </layout>
    </appender>
    <root>
      <level value="ALL" />
      <!-- If the following line is not included the log file will not be created even if log4net is configured with this file. -->
      <appender-ref ref="TestAppender" />
    </root>
  </log4net>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
    </startup>
</configuration>