Use @Option together with task closure

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1

I am trying to use @Option to expose a custom command line option for configuring a JavaExec task. I attempted to do it like this in my build.gradle file:

class MyJavaExec extends JavaExec {
  @Input
  @Option(description="Configure XYZ")
  def String xyz;
}
task runXyz(type: MyJavaExec) {
  systemProperties += ['custom.xyz': xyz]
  main = 'com.MyMain'
}

I then execute the task with gradle runXyz --xyz=something
The problem is that this ends up setting my custom.xyz system property to null. How can I do this to get hold of the value set with the command line option?