Some people have this problem...
From the README:

Q: I can't type \ or : in DOSBox.
A: This is a known problem. It only occurs if your keyboard layout isn't US.
Some possible fixes:
1. Switch your keyboard layout.
2. Use / instead.
3. Open dosbox.conf and change usescancodes=false to usescancodes=true.
4. Add the commands you want to execute to the "configfile".
5. Start the keymapper (CTRL-F1 or add -startmapper switch to DOSBox).
6. Use ALT-58 for : and ALT-92 for \.
7. for \ try the keys around "enter". For ":" try shift and the keys
between "enter" and "l" (US keyboard layout).
8. Use keyb.com from FreeDOS (http://projects.freedos.net/keyb/).

Also try setting usescancodes=TRUE or usescancodes=FALSE in dosbox.conf.

That's sounds complicated 😜

Use ALT-58 for : and ALT-92 for \

It doesn't seem that complicated to me. In Spanish keyboards, : is obtained by using shift+ñ (n with an ~ symbol above it). Try similar combinations on you non-us keyboard layout.

I fixed it late on and it worked 😊

what do you mean whit ALT-58?

press (and keep pressed ) alt
then type 58 on the numeric keyboard.
release alt