counter must be an item counter for an item in the project. The counter can be found in URLs like https://rollbar.com/myaccount/myproject/items/456/ (456 is the counter).

The success response is a 301 redirect like this:

HTTP/1.1 301 Moved Permanently
Location: /api/1/item/272505123

{
  "err": 0,
  "result": {
    "itemId": 272505123,
    "path": "/api/1/item/272505123",
    "uri": "/api/1/item/272505123"
  }
}