How can I format DateTime to web UTC format?

string foo = yourDateTime.ToUniversalTime()
                         .ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fff'Z'");

Why don't just use The Round-trip ("O", "o") Format Specifier?

The "O" or "o" standard format specifier represents a custom date and time format string using a pattern that preserves time zone information and emits a result string that complies with ISO 8601. For DateTime values, this format specifier is designed to preserve date and time values along with the DateTime.Kind property in text. The formatted string can be parsed back by using the DateTime.Parse(String, IFormatProvider, DateTimeStyles) or DateTime.ParseExact method if the styles parameter is set to DateTimeStyles.RoundtripKind.

The "O" or "o" standard format specifier corresponds to the "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK" custom format string for DateTime values and to the "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffzzz" custom format string for DateTimeOffset values. In this string, the pairs of single quotation marks that delimit individual characters, such as the hyphens, the colons, and the letter "T", indicate that the individual character is a literal that cannot be changed. The apostrophes do not appear in the output string.

The O" or "o" standard format specifier (and the "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK" custom format string) takes advantage of the three ways that ISO 8601 represents time zone information to preserve the Kind property of DateTime values:

public class Example
{
   public static void Main()
   {
       DateTime dat = new DateTime(2009, 6, 15, 13, 45, 30, 
                                   DateTimeKind.Unspecified);
       Console.WriteLine("{0} ({1}) --> {0:O}", dat, dat.Kind); 

       DateTime uDat = new DateTime(2009, 6, 15, 13, 45, 30, 
                                    DateTimeKind.Utc);
       Console.WriteLine("{0} ({1}) --> {0:O}", uDat, uDat.Kind);

       DateTime lDat = new DateTime(2009, 6, 15, 13, 45, 30, 
                                    DateTimeKind.Local);
       Console.WriteLine("{0} ({1}) --> {0:O}\n", lDat, lDat.Kind);

       DateTimeOffset dto = new DateTimeOffset(lDat);
       Console.WriteLine("{0} --> {0:O}", dto);
   }
}
// The example displays the following output: 
//    6/15/2009 1:45:30 PM (Unspecified) --> 2009-06-15T13:45:30.0000000 
//    6/15/2009 1:45:30 PM (Utc) --> 2009-06-15T13:45:30.0000000Z 
//    6/15/2009 1:45:30 PM (Local) --> 2009-06-15T13:45:30.0000000-07:00 
//     
//    6/15/2009 1:45:30 PM -07:00 --> 2009-06-15T13:45:30.0000000-07:00

string.Format("{0:yyyy-MM-ddTHH:mm:ss.FFFZ}", DateTime.UtcNow)

returns 2017-02-10T08:12:39.483Z