curl --request POST \
  --url https://api.reftree.co/campaigns/{campaignId}/participants \
  --header 'Content-Type: application/json' \
  --data '
{
  "username": "<string>"
}
'

{
  "id": "<string>",
  "campaignId": "<string>",
  "type": "idle",
  "username": "<string>",
  "origin": "facebook"
}