To create a rawbox in Target you just fire a manual mbox request from your browser. The sample request URL from the help doc Shruthi shared is a template you can use (simplified a little):
You need to update 3 things first though. Replace the 2 instances of "client_code" with your Target client code (find this in your at.js or mbox.js listed as clientCode='yourclientcode'). Then replace the "mbox_name" with what you want the name of the rawbox to be (e.g., eml_rawbox, cmp_email). Then simply fire the url in your browser. After loading the page will probably say something like file not found. This is okay as this was just a manual request to create the rawbox. Then in the Target UI you should see your new rawbox show up in the location dropdown though it may take a few minutes to show up the first time.
If I want add a new rawbox, is it needed to change the value of the parameter mboxSession (139603 2094853 in this explanation)? Or I just need to use a new rawbox name to replace former one in this script directly?