In a new plugin, depending on workflow-durable-task-step-plugin, I've extended DurableTaskStep:

public class MyCustomStep extends DurableTaskStep {
  ...
}

However, when trying to use the step via Pipeline Syntax, I'm getting the following error:

<st:include> No page found 'config-details' for class my.pkg.name.MyCustomStep

It seems it refers to DurableTaskStep/config.jelly.
How do I correctly extend that base class?