The directions at that site are pretty horrible, but the capability it offers is really great and has worked fine for me.
I do it by right clicking the start, selecting "Directions from here." Then right click the destination and select "Directions to here." Then I play with the routing to make it go my way, do the thing about moving the start and destination ever so slightly, then click "Link to this page" and copy the resulting URL that pops up in the pink box.
saddr is one of the components of the URL that google maps produces after you have laid out an itinerary and clicked "Link to this page." It's located at about position 46 in the URL and specifies the starting address. Then there will be a daddr, which is the destination or the first waypoint (if any). Every location after that will be indicated by "+to:"
If you aren't getting a URL like this, then something has gone wrong in setting up the itinerary. Are you sure you started out at
http://maps.google.com/maps?f=d ?
From what I've been able to gather by analyzing the URLs, the "important" step of going back to start and destination and moving them ever so slightly appears to involve google providing proper nomenclature for the waypoints. In the URLs that I've looked at, the locations appear to be OK with or without this step.
The itinerary that google produces is actually a pretty minimal one. It contains just enough information for google to be able to accurately reproduce it in the future according to its own routing algorithm. There is no guarantee that TomTom will provide the identical routing between waypoints, as it might have its own idea about things. You might have to go back and forth a few times before you nail it down sufficiently that it comes out the same in both.
Hang in there and keep at it. This capability is ever so much easier than doing it directly on the TomTom.