OK-ACCESS-TIMESTAMP The timestamp of your request.e.g : 2020-12-08T09:08:57.715Z

The timestamp value is the same as the OK-ACCESS-TIMESTAMP header with millisecond format of ISO, e.g. 2020-12-08T09:08:57.715Z.

var now = DateTime.UtcNow;
var timestamp = $"{now:yyyy-MM-dd}T{now:hh:mm:ss.fff}Z";

var timestamp = $"{DateTime.UtcNow:o}Z";

var timestamp = $"{DateTime.UtcNow:s}Z"

var timestamp = (DateTime.UtcNow.ToUnixTimeMilliSeconds() / 1000.0m).ToString(CultureInfo.InvariantCulture);