Error while post call on https://api.kite.trade/session/token

getting below error
{"status":"error","message":"`api_key` should be minimum 6 characters in length.","data":null,"error_type":"InputException"}

on Post call
{
"api_key": "xxx",
"request_token": "xxx",
"checksum": "xxx"
}

Status Code: 400 Bad Request