You can't necessarily do this (I experimented a lot with a separate "SSO" module that would post to the DNN login -- actually got it to work on 4.4, I think, but the newer versions don't work) for some reason.  I came up with an alternative method and packaged it with several other features in a module called SW.AutoLogon;  your approach could be implemented using the "querystring" method (which works doubly with HTTP Posts)... I offer a free trial so feel free to take a look at it.

*EDIT* I got it to work all the way through 4.5.3, but with 4.5.4 (.5, since .4 was quickly replaced) no longer works... likely due to the SSL stuff.  If you want to see the way to do that with a form that posts to the actual DNN login page, let me know -- otherwise my module works as a good "way around" it.