Calling a rest api with username and password - how to

If the API says to use HTTP Basic authentication, then you need to add an Authorization header to your request. I'd alter your code to look like this:

    WebRequest req = WebRequest.Create(@"https://sub.domain.com/api/operations?param=value&param2=value");
    req.Method = "GET";
    req.Headers["Authorization"] = "Basic " + Convert.ToBase64String(Encoding.Default.GetBytes("username:password"));
    //req.Credentials = new NetworkCredential("username", "password");
    HttpWebResponse resp = req.GetResponse() as HttpWebResponse;

Replacing "username" and "password" with the correct values, of course.

You can also use the RestSharp library for example

var userName = "myuser";
var password = "mypassword";
var host = "170.170.170.170:333";
var client = new RestClient("https://" + host + "/method1");            
client.Authenticator = new HttpBasicAuthenticator(userName, password);            
var request = new RestRequest(Method.POST); 
request.AddHeader("Accept", "application/json");
request.AddHeader("Cache-Control", "no-cache");
request.AddHeader("Content-Type", "application/json");            
request.AddParameter("application/json","{}",ParameterType.RequestBody);
IRestResponse response = client.Execute(request);