Go to your Steam launch options and put: -map (mapname)

Where mapname is:

01 – Seppuku Station – $BULLET.MAP
02 – Zilla Construction – $DOZER.MAP
03 – Master Leep’s Temple – $SHRINE.MAP
04 – Dark Woods Of The Serpent – $WOODS.MAP
05 – Rising Son – $WHIRL.MAP
06 – Killing Fields – $TANK.MAP
07 – Hara-Kiri Harbor – $BOAT.MAP
08 – Zilla’s Villa – $GARDEN.MAP
09 – Monastery – $OUTPOST.MAP
10 – Raider Of The Lost Wang – $HIDTEMP.MAP
11 – Sumo Sky Palace – $PLAX1.MAP
12 – Bath House – $BATH.MAP
13 – Unfriendly Skies – $AIRPORT.MAP
14 – Crude Oil – $REFINER.MAP
15 – ♥♥♥♥♥♥ Mines – $NEWMINE.MAP
16 – Subpen 7 – $SUBBASE.MAP
17 – The Great Escape – $ROCK.MAP
18 – Floating Fortress – $YAMATO.MAP
19 – Water Torture – $SEABASE.MAP
20 – Stone Rain – $VOLCANO.MAP
21 – Shanghai Shipwreck – $SHORE.MAP
22 – Auto Maul – $AUTO.MAP

You can also use /s1-4 to set your skill level.

example: -map $YAMATO.MAP /s1
Will load flying fortress on tiny grasshopper difficulty on starting the game

I'm not entirely sure on the DLC maps but launching directly into the DLCs using the above method should send you to the following maps

01 – Chinatown – $WHIRL.MAP
02 – Monastery – $TANK.MAP
03 – Trolley Yard – $BOAT.MAP
04 – Restaurant – $GARDEN.MAP
05 – Skyscraper – $OUTPOST.MAP
06 – Airplane – $HIDTEMP.MAP
07 – Military Base – $PLAX1.MAP
08 – Train – $BATH.MAP
09 – Auto Factory – $AIRPORT.MAP
10 – Skyline – $VOLCANO.MAP
11 – Redwood Forest – $SHORE.MAP
12 – The Docks – $AUTO.MAP

Similar to wanton destruction

01 – Wang's Home– $WHIRL.MAP
02 – City of Despair– $TANK.MAP
03 – Emergency Room – $BOAT.MAP
04 – Hide and Seek – $GARDEN.MAP
05 – Warehouse – $OUTPOST.MAP
06 – Military Research Base – $HIDTEMP.MAP
07 – Toxic Waste – $PLAX1.MAP
08 – Crazy Train– $BATH.MAP
09 – Fishing Village – $AIRPORT.MAP
10 – The Garden – $REFINER.MAP
11 – The Fortress – $NEWMINE.MAP
12 – The Palace – $VOLCANO.MAP
13 – Prison Camp – $SHORE.MAP

Hope this helps you when finding secrets or fortune cookies