Engagespot API returns invalid identifier when I send a notification

I am trying to create one to one user notification system using PHP and Engagespot. The user is logged in and I called Engagespot.identifyUser() function. After that If I send notification to this identifier it says invalid identifier.

This is my API call.

POST /2/campaigns HTTP/1.1
Host: api.engagespot.co
Api-Key: XXX

{
   "campaign_name":"Test Campaign",
   "notification":{
       "title":"Hey XYZ, Message only for you.",
       "message" : "How are you",
       "icon":"http://mywebsite.com/icon.png",
       "url":"https://mywebsite.com"
   },
   "send_to":"identifiers",
    "identifiers" : ["daniel"]   
}

Any idea why this doesn't work?