Link to page

Schema:

{
  "type": "object",
  "properties": {
    "type": {"type": "string", "pattern": "^form-field$"},
    "id": {"type": "string", "pattern": "^[0-9a-f]{8}-([0-9a-f]{4}-){3}[0-9a-f]{12}$"},
    "machine_name": {"type": "string"},
    "data": {
      "type": "object",
      "properties": {
        "uid": {"type": "string","pattern": "^form-link$"},
        "title": {"type": "string"},
        "value": {
          "type": "object",
          "properties": {
            "url": {"type": "string"}
          }
        }
      }
    }
  },
  "required": ["id", "type", "data"]
}

Example output:

Given "Link to page" form element on a component set up like so

 

And used on Node layout canvas like so (given that "Test Page, Please Ignore" is a Drupal Node with id "5")

JSON:API output for this form field will be this:

{
  "type": "form-field",
  "id": "7cc6c4c8-da14-4c2d-8e4d-f34187758f43",
  "machine_name": "link-to-page",
  "data": {
    "uid": "form-link",
    "title": "Link to page",
    "value": {
      "url": "https://sitestudiodocs.acquia.com/node/5",
      "jsonapi_link": "https://sitestudiodocs.acquia.com/jsonapi/node/layout_page/050cdf6a-310f-49b8-8059-5e0e705fb961"
}
  }
}

The "value" property is an object containing "url" property that is absolute link to "Test page, Please Ignore" page with Drupal node id 5. The "jsonapi_link" here is canonical Drupal route to JSON:API resource representing "Test Page, Please Ignore" node. 

If same component containing "Link to page" form element is used with external URL like so:

JSON:API output for this form field will be this:

{
  "type": "form-field",
  "id": "618bd7fe-e48d-4407-b1b3-dfae708b9d75",
  "machine_name": "link-to-page",
  "data": {
    "uid": "form-link",
    "title": "Link to page",
    "value": {
      "url": "https://youtu.be/dQw4w9WgXcQ?si=ELei-LXbyEZU1Lcd"
    }
  }
}

The "value" property is an object containing "url" property that contains external link. As there is no internal Drupal entity to link to there is no "jsonapi_link" property.