How do I send event_id with Facebook pixel?

I get an error from Facebook:

Purchase Event Missing Some Deduplication Parameters You're sending

Purchase events through both your pixel and the Conversions API, butthey're not being deduplicated properly because not enough of theseevents are receiving an event_id parameter. Event instances sentwithout an event_id parameter cannot be deduplicated.

Add an event_id parameter to all Purchase events that you're sendingfrom both your pixel and the Conversions API. To add an event_idparameter to the events you're sending through the Conversions API,you can use the Payload Helper tool on the Facebook for Developerssite to validate that your payload is set up correctly. To add anevent_id parameter to the events you're sending through your pixel, gointo your website's source code and add an event_id parameter to eachof your Purchase event instances.

But I am sending event_id in my pixel event for Purchase!