The composition would have to know where to go on the timeline when it is opened from a link. Obviously, this information has to be sent with the html request. A way to do this is to include this information in a query line (after a "?" at the end of the URL). You will then have to have a script in your composition that will test for query lines and parse the content. Simply sending a time value for the position you want your composition to open at will enable you to use the play() function: i.e., play(yourStartTimeValue).