You have an error on your namespace you are using a fixed url:
<WebService(Namespace:="http://localhost:51561/APIServer/")> _
if you used dynamic the url should be:
<System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
You should use dynamic url binding or change the localhost:51561 to WEBSITENAME:{port}
But i would choose the dynamic way than you don't have to worry about this. You can set this in properties of your project.
I have a nice tutorials from codeproject here:
[
How-to-make-your-Web-Reference]
if you not calling a dynamic url you need to set the url in your application before you call it.
This should fix your 500 error.
The best way to use you webservice is just add the url to your project under reference, will make it a lot easier for you instead of using a WebRequest.
Just go to your project and say: add refenerce => service reference. Past down your url and you should be able to use it in your project with the result and comments etc.
Here a example how to use your soup service when added:
Dim service As New ServiceReference1.Service1SoapClient
Dim test = service.TestRest()