Launch android app from a website

I'm currently working on a website, and I need to connect it to an external app, which I don't own, and can't modify.(in this particular case is QR Droid Zapper) It doesn't mind if it requires html, php, js or something else. I don't need to pass any data, just launch it with the default settings, like if a desktop shortcut where used.

Is it posible to launch the android app from a website link or button? Let's say something like(seudocode):

To open DemoApp <a href="open:demoapp-url">Open DemoApp</a>

If it's posible. Which is the method to do this?