How can I launch a modern game with DOSBox
You (probably) cannot.
DOSBox is for an era of games that are around Windows 98 or earlier. Basically games that do not use any of the OS layer of modern operating system (we're talking about everything from mouse to sound to graphics).
For your specific question, since Starcraft 2 requires at least Windows XP, it definitely won't work with plain DOSBox. I guess, in theory, you can get DOSBox to run Windows XP, but it'll be the dumbest setup I've ever seen...