Facebook send api - Unable to send a message using user id

Question

When I execute the following request from https://developers.facebook.com/docs/messenger-platform/send-api-reference

curl -X POST -H "Content-Type: application/json" -d '{
  "recipient":{
    "id":"USER_ID"
  },
  "message":{
    "text":"hello, world!"
  }
}' "https://graph.facebook.com/v2.6/me/messages?access_token=PAGE_ACCESS_TOKEN"

I get this error:

Graph returned an error: (#100) Parameter recipient[id]: Invalid id: "the id of the recipient"

The documentation states that:

The id must be an ID that was retrieved through the Messenger entry points or through the Messenger webhooks (e.g., a person may discover your business in Messenger and start a conversation from there. These IDs are page-scoped IDs (PSID). This means that the IDs are unique for a given page. If you have an existing Facebook Login integration, user IDs are app-scoped and will not work with the Messenger platform.

I don't understand where to get the id from. Can someone explain how to obtain the id to use in the API?

Answer 1

You get the userID from the webhook when a user messages you. This ID is not the ID of the user, instead every user gets assigned a unique ID when it enters your page. This ID is then used to communicate between your page and the user.