How To: Get embed token using Get/Post only

Step 4 is not working for me. I was able to get the access token in step 2 and then able to run step 3. For step 4 I entered the header information into postman under header and the data information under the body but I am getting this error:

{
"error": {
"code": "BadRequest",
"message": "Bad Request",
"details": [
{
"message": "Unexpected character encountered while parsing number: W. Path '', line 1, position 6.",
"target": "request"
}
]
}
}
]
}
}

Is anyone else experiencing this or know of a way I can fix this? Also for reference I am doing a server-web app and have added the client secret into the first step, not sure if I would need to add anything in this 4th step too or if there is another reason it is showing this error.