It's somewhere a 'shifted' key, on my layout (azerty be) it's shift+%.
..... or you can set your keybaord layout to your own with the command
keyb xx
For me it's
keyb be
For you .......
http://vogons.zetafleet.com/viewtopic.php?t=21824
Or set a key to " with dosbox keymapper ..... (-> dosbox manual)