Unable to get auth token

Im using postman to post " api-key, request-token and checksum"
to https://api.kite.trade/session/token. But getting this response. Please help

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

I also used the header as X-Kite-Version:3