My web service operation call only opens test page in IIS - I am not sure how to call the operation to return my xml

I am new to web services.

I have built a web service in C# that consumes a third party service and then returns the XML response from that call in a web method.

When I test this in IIS on the local web server it works perfectly.

However when I try to call or invoke the service via the URL I use on the test page I can't return a response.

If I use a GET I simply get the test page loading and the message I can't use this test page outside of the local server.

In short, I simply want to know how to call the web method externally and mimic the INVOKE button being pressed so I get the response passed back.

This is probably really simple but I can't get my head around it.

In addition if I use Postman to try and call the URL:

WebService.asmx?op=GetSalesOrders

I get the error message detailed below which is a step forward but I am still unsure about how to package up a call to this service