For starters, you’ll need a different cut of beef. The beef in Italian Beef sandwiches is sliced (at least it starts out that way). Unless I’m misunderstanding the size and nature of a pot roast, the best you can hope for is shredded beef. So right off the bat, the texture will be off. You can probably still make a pretty good tasting sandwich, but it won’t be a Chicago Italian Beef.
I think most places are using some form of roast beef (top round, top sirloin butt, eye of round, etc.). It needs to be fully cooked, then cooled before slicing very thin. The sliced beef goes into the au jus for about 15 minutes, the piled up on crusty bread. Personally, I like the whole sandwich then dipped into the au jus with hot peppers (jardiniere). Very messy, but absolutely delicious!
You can piece together some nice tips in these videos, including a specific recipe for the au jus: