Create a Meta custom conversion and attach it to a pixel event via the Marketing API

Steps

1. Create a custom conversion via POST /{ad_account_id}/customconversions with a name, pixel_id, and rule; the rule is a JSON filter object that matches pixel events by event name and optional URL or parameter conditions.
2. Set the custom_event_type to one of Meta's standard categories (e.g., PURCHASE, LEAD, ADD_TO_CART) to enable reporting in standard conversion columns; use OTHER if the event does not map to a standard type.
3. Optionally set a value_type to CONSTANT (and supply a custom_event_values array) or USE_VALUE if your pixel fires a currency value; this controls how conversion value is attributed.
4. Reference the custom conversion ID in the ad set's promoted_object.custom_conversion_id to optimize campaigns toward it; ensure the ad set's optimization_goal is OFFSITE_CONVERSIONS.
5. Verify the custom conversion is receiving data by checking its status via GET /{custom_conversion_id}; the last_fired_time field shows the most recent pixel event that matched the rule.
6. Test the rule logic using the Meta Pixel Helper browser extension before going live; incorrect URL match conditions are a common cause of zero-firing custom conversions.