How to customize schematics schema.json?

Question

Is it possible to customise Angular schematics schema.json? I wanna add extra properties to the schema. In other words, I would like top extend the schematics schema.json syntax. How to do that?

{
  "$schema": "http://json-schema.org/schema",
  "$id": "SomeId",
  "title": "Some",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "minLength": 1,
      "description": "Name of something",
      "default": "",
      "MyExtraProp": 12
      "x-prompt": "Some prompt"
    }
  },
  "required": ["name"]
}

I created my own schematics CLI which is based on the reference implementation:

https://github.com/angular/angular-cli/blob/main/packages/angular_devkit/schematics_cli/bin/schematics.ts

How can I make schematics to actually recognise "MyExtraProp" and pass it to my PromptProvider?

function _createPromptProvider(): schema.PromptProvider {
  return (definitions) => {
// definitions contains schema.PromptDefinition array. But my custom prop is not part of it, it's ignored.
...
}