How can i get to command prompt in dos through bios?

Sorry but basically you cant access command propmt through the bios. The command prompt is part of the operating system of the pc, either as DOS (disk operating system) or part of windows.
The BIOS (Basic Input output system) which is on a rom(read only memory) chip on the mother board, and initiates post(power on self test) when the computer starts, without the bios the computer would not boot up at all. To access the command prompt you must have a operating system installed, ie Windows. hope this helps.