To allow registration, you need to run JavaScript similar to the following:

navigator.registerProtocolHandler('webcal', 'https://www.site.com/page.asp?q=%s', 'App Name');

In this case, whenever there is a href that begins with webcal: the parameters are passed to the site using the %s macro substitution variable.

Here's an example anchor/link

So...  When the protocol handler is registered, when you click on a URL that starts with "webcal:", the %s gets replaced with the entire webcal://blablabla.

That allows the called website to do the URL GET to get the ICS.

Here's a list of common prefixes used right now: