Originally posted by the ROM Map
$009CB1 | 1 byte | Level number | Determines the low byte of the intro level number. Subtract 0x24 for the actual low byte (by default, E9-24=C5, meaning level C5, or possibly 1C5 with Lunar Magic's Display Level Message 1 fix). Change to  to skip the intro level and warp to the overworld immediately.
Save this as an .asm and patch it: