Request Body format to make a Service Catalog REST API call for 'Buy Item' with variables

POST https://<instance_name>.service-now.com/api/sn_sc/servicecatalog/items/<sys_id>/order_now
{
"sysparm_quantity": "1",
"variables":
{
"requested_for":"<sys_id_of_user>",
"short_description":"short description",
"description":"Test"
}
}