I don't think you realise how big a question you are asking.
The reality is, this is not really viable using Web Designer unless you are already knowledgeable in this area and are willing to fiddle a lot to make it work.. Placeholders are fine for code snippets, widgets and other implants to add extra functions and tricks to your site. But what you are asking is a whole different ball game and not suitable for a web design application such as Web Designer.
Explaining how to setup an SQL database at your hosting account cpl, then write the code tie it to input fields in your page that connect back to the db and then return results to the page etc isn't really a Xara Products related topic, it would also be a lot to ask of someone. It's not a simple thing.
Whether ASP or PHP (windows or linux servers), it's much the same thing, though linux is much more common of course.

I'd suggest you go to a fully functional CM system like WordPress or Joomla and the like. You can still use Xara to design your graphics and even prototype a layout.
Take the new XaraXone for example, this is a WordPress site.