Right now entering a purchase link is required. If your customers won't be buying your software then you can just put your company webpage.
Also, there's no way to disable the buy button in the TurboActivate wizard. You don't have to use the TurboActivate wizard, though. Using the TurboActivate API you can build a wizard that fits your needs. Or you can just use the translation ability to rename the buy button to something like "Get help here" with a link to your support page.