From the guide:

"For the technically minded, the tool has been developed using the ABN Lookup web services which  provides a SOAP/XML interface to the information stored in ABN Lookup via the internet"

Unfortunately, at the moment the Http Web Request activity does not formally support SOAP. That said, it could be made to work in some cases, but even then it might not be quite the "out of the box" experience you are looking for. If they happen to provide a REST API, then it will be much easier to integrate the Http Web Request activity with this web service... though it does not seem like they do, from the article.

Even then, the article does not provide any info about the interface that you need in order to make the API calls (the tool they describe seems to use such an interface internally, though the guide does not provide it). The article does mention that you can get access the interface by registering and asking for it, I suppose. You'd need to do that first... though I have no promises as to whether or not we'll be able to do much with SOAP. But if you are adventurous, you could try digging a bit deeper =)