If this property is not null when you call Sign, an X.509 RFC 3161 time-stamped
signature will be created by using the service provided in the URL.
Note that this property serves as an option for Sign, and is not persisted in the PDF
If your time-stamping service provider requires authentication,
you can specify your credentials in the Uri. For example,
ABCpdf uses System.Net.WebRequest to send the time-stamping
request. You can use
to customize trust relationship establishment when you connect to
an SSL/TLS channel.