You can create an "mbox" or location in Target by simply adding an mbox to a page and loading that page in a browser. Alternatively, you can fire an mbox request in your browser to create an mbox in your Target account. Here is a sample mbox request you can modify to create an mbox/rawbox for your Campaign purposes. Replace CLIENT with your Target account client code. Replace MBOX with the name you choose for your mbox/rawbox - it can be what ever you choose so long as it's alphanumberic.
You should get a json response that returns your mboxSession id plus a tntId.