The next map vote would be like this: "/callvote nextmap map_name-b4" You can also add a reason to it if you wish (the server may not support adding reasons).
Some servers have !listmaps. A command that displays a list of the maps contained within the server's /base folder. If the server doesn't have that, then you can also see a list of maps by pressing "Esc" and clicking "Vote", "nextmap", etc.