...
// 1. Import the CodePush namespace
using CodePush.ReactNative;
...
class MainReactNativeHost : ReactNativeHost
{
    // 2. Declare a private instance variable for the CodePushModule instance.
    private CodePushReactPackage codePushReactPackage;

    // 3. Update the JavaScriptBundleFile property to initalize the CodePush runtime,
    // specifying the right deployment key, then use it to return the bundle URL from
    // CodePush instead of statically from the binary. If you don't already have your
    // deployment key, you can run "code-push deployment ls <appName> -k" to retrieve it.
    protected override string JavaScriptBundleFile
    {
        get
        {
            codePushReactPackage = new CodePushReactPackage("deployment-key-here", this);
            return codePushReactPackage.GetJavaScriptBundleFile();
        }
    }

    // 4. Add the codePushReactPackage instance to the list of existing packages.
    protected override List<IReactPackage> Packages
    {
        get
        {
            return new List<IReactPackage>
            {
                new MainReactPackage(),
                ...
                codePushReactPackage
            };
        }
    }
    ...
}