Hi @Chandra31 

Endpoint URL: Use the Service Catalog API's order_now endpoint.
https://<your-instance>://<catalog_item_sys_id>/order_now

Try with Payload

{
  "sysparm_id": "a96250e407d01410540bf2508c1ed014", // pass the  sys_id of your Catalog Item
  "sysparm_quantity": "1",
  "variables": {
    "variable_name_1": "variable_value_1",
    "variable_name_2": "variable_value_2",
    "requested_for": "1f78df4047bc4bcbf6" // For reference fields like 'requested_for', you need to pass the sys_id of requested for  }
}

Use correct credential by which you can create request in instance itself.