Without the actual values you're using for the surl and other variables, or at least the actual value of completeurl, it will be hard to tell you the exact cause of your error. This code creates the URL correctly provided the correct information. If you want to email me this information rather than post it here, feel free to send it over to our webservices support team. While looking at your code I see what I think is an unrelated issue to the 404 error. Your XML you're creating is incomplete and will not work. You need to use the full XML string builder he referenced:
data.Append(@"<entry xmlns=""http://www.w3.org/2005/Atom"">");
data.Append(@"<id>data:,</id>");
data.Append(@"<title/>");
data.Append(@"<author/>");
data.Append(@"<updated>2008-06-30</updated>");
data.Append(@"<content type=""application/vnd.ctct+xml"">");
data.Append(@"<ContactList xmlns=""http://ws.constantcontact.com/ns/1.0/"">");
data.Append(@"<OptInDefault>false</OptInDefault>");
data.Append(@"<Name>" + listname + "</Name>");
data.Append(@"<SortOrder>92</SortOrder>");
data.Append(@"</ContactList>");
data.Append(@"</content>");
data.Append(@"</entry>");
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