API V2: tokens POST request requires transientKey

Hi!
When I try to use VanillaForum API v2 POST request 'tokens' (https://docs.vanillaforums.com/help/apiv2/swagger/#/Tokens/post_tokens) I unexpectedly get the error 400 "transientKey is required":

{
    "message": "transientKey is required.",
    "status": 400,
    "errors": [
        {
            "field": "transientKey",
            "code": "missingField",
            "message": "transientKey is required."
        }
    ]
}

The same problem appears when I send a test request (via "Try it out" button) from API v2 documentation on VanillaForum Settings Page. I change nothing in this test request:

curl -X POST "http://t103.lan/api/v2/tokens" -H  "accept: application/json" -H  "Content-Type: application/json" -H  "x-transient-key: EGWKXeQXNqHkjJog" -d "{  \"accessTokenID\": 0,  \"name\": \"Unknown Type: string,null\",  \"accessToken\": \"string\",  \"dateInserted\": \"2018-09-04T09:41:26.098Z\"}"


Error: Bad Request
Response body

{
  "message": "transientKey is required.",
  "status": 400,
  "errors": [
    {
      "field": "transientKey",
      "code": "missingField",
      "message": "transientKey is required."
    }
  ]
}

The problem doesn't appear for a couple of other POST requests I tried. Definitely, I don't pass non-documented transientKey.

1. Is this a bug?
2. Is there a workaround or quick patch for v2.6.1?
3. Could you please show me an example of request (and information how to determine transientKey) if I do something wrong and transientKey is really needed?

Thanks a lot,
Anvar