Question on Isilon API "/platform/4/auth/access/"

Hey guys...  looking for details on the "format" of .  I got it to work with "SID:xxxxxxxxxxxxxxxxxx", but want it to work with {DOM}\{ID} (i.e. nam\mz55p7...)  I was told by a teammate that you could not use it with a username and that I had to lookup the SID for the user.  That kinda seems like overkill.  The API docs are really generic and few examples.  Can anyone help answer this???

Thanks!

Chris Thomas