Schlüssel means "key"
there is no way to give you a key in SH3 other than to - rather unrealistically - include the key in the message itself
which of course would be very stupid in real life
what you will want to do is google search "Enigma Simulator"
review the instructions on how to use it. Appears that there is a 3 rotor key "B J L" for this message
Set the rotors to match the key, then it should just be a matter of typing the coded text into the enigma sim, and out comes the plain text version.