Configure a plugin that uses contianers from another plugin

I am trying to create a plugin that uses another plugin. I have figured out how to apply the plugin but I cannot figure out how to configure the plugin. I can configure it in my build.gradle. I want to use the configuration that is currently in the build file into the plugin. Eventually I will also allow the user to overwrite this configuration.
The scenario is that we have a bunch of projects that all have multiple levels of test. I want to add a test set that by default is call systeTest and is under src/SystemTest. Instead of creating the source set myself I want to use the org.unbroken-dome.test-sets plugin.

In my build.gradle I have
testSets {
systemTest {dirName = “systemTest”}

In my plugin I have figured how to apply the plugin but not how to configure it.