How to get form fields in Facebook Lead Ads API?

You can get these by adding non-default field qualifiers, so the url becomes /<form_id>?fields=id,name,qualifiers.

I haven't found it anywhere in the official docs though. But it is quite common for new FB features to be poorly documented.

Example output

{
  "id": "1234567890000",
  "name": "test form",
  "qualifiers": [
    {
      "field_key": "first_name",
      "label": "First Name",
      "question": "First name",
      "id": "777888999000111"
    },
    {
      "field_key": "last_name",
      "label": "Last Name",
      "question": "Last name",
      "id": "111222333444555"
    }
  ]
}

Just a warning since this answer comes first on google search.

Since Facebook API v5.0 field "qualifiers" is removed and will throw an error.

Replace it with "questions" which is similar (if not exact) syntax as qualifiers. Found out the hard way on production server...