REST API Explorer: I have to give the POST method url for submiting the catalog item request form

Hi Team,

I have a requirement to give the POST HTTP link for submitting the catalog item request form,
I have tried using:
Namespace: sn_sc
API Name: Service Catalog API

Buy Item (POST)
sys_id: (Given sys_id of catalog item)
In Raw Data:
{
"sysparm_quantity": "1",
"requested_by": "sys_id of user",
"requested_for": "sys_id of user",
"discription": "API test",
"assignment_group": "sys_id of group"
}
}

After including the mandate variables value as it throwing an error messge: 

Mandatory Variables are required

POST HTTP: 

POST https://dev.service-now.com/api/sn_sc/servicecatalog/items/catalog_item_sys_id/order_now

{
  "error": {
    "message": "Mandatory Variables are required",
    "detail": ""
  },
  "status": "failure"
}

It would be helpful, if any suggestion provided to send the correct variable format.