What parameters are required to create an “Add to Google Calendar” link?

Explanation about the available parameters:

anchor address:
http://www.google.com/calendar/event?
This is the base of the address before the parameters below.


action:
    action=TEMPLATE
    A default required parameter.
    
src:
    Example: src=default%40gmail.com
    Format: src=text
    This is not covered by Google help but is an optional parameter
    in order to add an event to a shared calendar rather than a user's default.
    
text:
    Example: text=Garden%20Waste%20Collection
    Format: text=text
    This is a required parameter giving the event title.
    
dates:
    Example: dates=20090621T063000Z/20090621T080000Z 
           (i.e. an event on 21 June 2009 from 7.30am to 9.0am 
            British Summer Time (=GMT+1)).
    Format: dates=YYYYMMDDToHHMMSSZ/YYYYMMDDToHHMMSSZ
           This required parameter gives the start and end dates and times
           (in Greenwich Mean Time) for the event.
    
location:
    Example: location=Home
    Format: location=text
    The obvious location field.
    
trp:
    Example: trp=false
    Format: trp=true/false
    Show event as busy (true) or available (false)
    
sprop:
    Example: sprop=http%3A%2F%2Fwww.me.org
    Example: sprop=name:Home%20Page
    Format: sprop=website and/or sprop=name:website_name

add:
    Example: add=default%40gmail.com
    Format:  add=guest email addresses

details: (extra)
    Example: details=Event%20multiline%0Adetails
    Format:  details=description text (google also accepts html in this text)

http://useroffline.blogspot.com/2009/06/making-google-calendar-link.html