If you are using Visual Studio, you can add a Service Reference to the external service. Visual Studio will create strongly-typed C# bindings to the reference. You will need the URL of the SOAP service to add it. See the Stack Overflow question here (the process is the same for Web Apps):
How to add a web service reference in Visual Studio 2015 to a console app
The link to the MSDN documentation for the process is here:
https://msdn.microsoft.com/en-us/library/bb628652.aspx
Once you have added the Service Reference, you can use the service by simply adding a using
directive containing the namespace you set when you added the service and using the bindings that Visual Studio made for you.
If you do not see Add Service Reference, you may be using a .Net Core project which may not have this ability.
Edited to add information on the specific service you linked:
using SmsTest.SmsService;
namespace SmsTest
{
class Program
{
static void Main(string[] args)
{
var msg = "Your SMS Message";
var serviceProviderId = 42;
// Use the constructor overloads for ServiceClient to configure how to connect
var service = new MyServiceClient();
service.smsSend(serviceProviderId, msg);
}
}
}
This is using the information you provided with my program's default namespace as SmsTest
and the Service Reference's namespace as SmsService
.