WAF custom rule block action (custom html)

Is it possible that “free tier” is missing the option to specify a custom html for WAF custom rules or am I doing something wrong here?

What is shown in custom rules:
image

What I would have expected to be shown instead (screenshot from “rate limiting rules” action):
(Sorry, I can’t upload 2 images since this is my first post. I’ll try to update it with a second image instead!)

Here is the second image:
image

I just tried to add a rule (with a custom block response) through the API (v4) and got the following response back:

{
  "result": null,
  "success": false,
  "errors": [
    {
      "message": "not entitled to use a custom response",
      "source": {
        "pointer": "/rules/1/response"
      }
    }
  ],
  "messages": null
}

To respond to my own question: Yes, “free tier” does not allow to add a custom block action.