The keyboardmapping will not always work ingame, so you're better of learning the keys in standard dosbox.
Two possibilities to easily resolve this problem:
-> or use / instead, does the same as \ in dosbox ( the / in numpadblock)
If you're on a laptop without numpadblock simply search for the / or the \ by hitting all keys, try first the ones besides left shift, that's where it usually can be found.
US non-shifted:
click ->
US shifted
click ->
