Append your key and the token obtained from either Access/exchange-token or Access/impersonate to the querystring (key=mykey&token=kj123khb123hb123hb123jh123) or to the headers (Authorization:Bearer kj123khb123hb123hb123jh123) of your request.

{
  field_configuration: [
    {
      key: "client_name",
      value_type: "string"
    },
    {
      key: "client_number",
      value_type: "numeric"
    }
  ]
}