Problem manipulating applicationId in Android Studio Gradle project

In an Android Studio project I have the following lines:

// do variant level customisation
android.applicationVariants.all { variant ->
    // adjust applicationId for Free variants

    if (variant.name.contains("Free")) {
        variant.mergedFlavor.applicationId += "_free"
    }
 }

This generates the following deprecation warning:

'MergedFlavor.getApplicationId()' is obsolete and has been replaced with 'VariantProperties.getApplicationId()'.

I have been unable to find an alternative syntax that will replace what I have and remove the deprecation warning. I am aware of "applicationIdSuffix" but this will always add a leading "." which I cannot have. Any thoughts much appreciated.