See the Annotations example project for a full example.
However the following is a code snippet showing how this might
be used.
var sig = doc.Form.AddDocTimestamp("Timestamp");
Uri uri = new Uri("http://timestamp.digicert.com");
Oid oid = new Oid(CryptoConfig.MapNameToOID("SHA256"));
sig.TimestampServiceUrl = uri;
sig.Timestamp(oid, 0);
Dim sig = doc.Form.AddDocTimestamp("Timestamp")
Dim uri As New Uri("http://timestamp.digicert.com")
Dim oid As New Oid(CryptoConfig.MapNameToOID("SHA256"))
sig.TimestampServiceUrl = uri
sig.Timestamp(oid, 0)