POST taxonomy item using Services module
Here is the JSON format to add taxonomy term. I've tested it and works perfect.

{
    "vid": "2",
    "name": "Term Name",
    "description": "your description"
}


Here vid: vocabulary ID

Make sure the current logged-in user has privileges to add taxonomy term. and make sure to add token in Headers like follows

name: X-CSRF-Token

value: your-token

To know your token access the following path

yourdomain/services/session/token