Look, a new group! I had no idea this was out there.
I suspect you've already gotten an answer to your question elsewhere, but I'll respond just in case.
If you direct your users to "root" of Tableau Server, than the best you'll be able to do is use the "Set default start page for all users" feature to show them the results of a search for the dashboard in question - it won't actually display the dashboard, however - your users will need to click the item in our search results.
What I'd suggest you try is giving people a "direct link" to your dashboard to click on. They'll still be prompted to login to Tableau Server, but after they do, your dashboard will immediately render. Use the following formula to do this:
Hope this helps!