I had a hard time finding a sample or any guidance for how to test out a scripted SOAP service.

Here is one method I found that works:

1. Download and install postman
2. Create a new request
   
   1. Set to POST
      
   2. Use URL: https://yourinstancename.service-now.com/yourscriptedwebservicename.do?SOAP
      
   3. Authorization: BasicAuth
      
      1. Specify your the SN account username and password you will use for the interface
         
   4. Headers
      
      1. Add a key “Content-Type” and set the value to “text/xml”
         
   5. Body

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:u="http://yourinstancename.service-now.com/yourscriptedsoapservicename">
    <soapenv:Body>
        <yourfunctionname>
            <yourinputparameter>hello world</yourinputparameter>            
        </yourfunctionname>
    </soapenv:Body>
</soapenv:Envelope>

Here is what that should look like: