Assuming you have exposed this web service through SharePoint URL, there is a way you can achieve this. You just need to locate the root web.config file and enable "HttpPostLocalhost" and/or "HttpPost" (if you want to invoke this web service remotely).
In SharePoint, web services usually are located in /_vti_bin/webservice.asmx. And the directory that maps to this path is by default: "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\isapi" (again, assuming it's SharePoint 2010).
Just open the web.config file and you'll see the following section as default values:
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