Facebook API lead forms for ads objective error for version 18

Since Facebook updated to version 18 I'm having troubles with adding lead ad forms using the API.

Reference: https://developers.facebook.com/docs/marketing-api/guides/lead-ads/create/

When creating campaigns using the objective LEAD_GENERATION it gives the following error:

"Objective LEAD_GENERATION is invalid. Use one of: OUTCOME_LEADS, OUTCOME_SALES, OUTCOME_ENGAGEMENT, OUTCOME_AWARENESS, OUTCOME_TRAFFIC, OUTCOME_APP_PROMOTION."

However changing the objective to OUTCOME_LEADS give the following error when creating the ad:

"Creative with lead form can only be used for Lead Generation objective. Please create a new one."

I had the same issue ,and the docs are not very clear. Try adding the "destination_type": "ON_AD" to the ad set params when creating seemed to solve the issue,

Example:

{
  "name": "Adgroup Name",
  "billing_event": "IMPRESSIONS",
  "optimization_goal": "LEAD_GENERATION",
  "campaign_id": 00000000000000,
  "bid_strategy": "LOWEST_COST_WITH_BID_CAP",
  "daily_budget": 2000,
  "targeting": {
    "custom_audiences": [
      111111111111
    ],
    "geo_locations": {
      "custom_locations": [
        {
          "latitude": "41.81",
          "longitude": "-88.07",
          "radius": 50
        }
      ]
    },
    "publisher_platforms": [
      "facebook"
    ]
  },
  "promoted_object": {
    "page_id": 222222222222222,
    "product_set_id": 333333333333333333
  },
  "bid_amount": 125,
  "destination_type": "ON_AD",
  "attribution_spec": [
    {
      "event_type": "CLICK_THROUGH",
      "window_days": 1
    }
  ]
}