Attach a simple custom driver to a web form

I am trying to create a custom web form controller. I would like to be able to attach my driver to any web form and have it show the message & # 39; xyz & # 39; when the form is finally sent (multi-step assistant). I have this file in my custom module in: my_module / src / Plugin / WebformHandler / PostWebformHandler.php.

addMessage (t (& # 39; xyz & # 39;));
}

}

When browsing the configuration of my web form, I see:

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but when I click Add handler for Test b Is not it attached to my forms handlers?

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