open the client in offline mode(start menu)

create a new baseplate

export level as .bopjson

close the client

open the .bopjson in a text editor

search "level_sky" and change the value to -1

save

reopen the level in the offline client

export as .bop (the .bopjson won't load in a level slot on the website for some reason)

close client

join a level slot on the site and load the exported .bop

you have won the game