I don't have the 390, but have you tried the usual Ctrl + R shortcut? Cruise Enable: Ctrl + R Cruise Disable: Ctrl + Shift + R I've found that implementation varies from train to train.
I created a small "button box" with Arduino, to give me the buttons I need. Not only do I not have room for a keyboard, but the main advantage of the Arduino is that I can program a button to send key sequences, like Ctrl-R and Ctrl-Shift-R. So, I have buttons for Cruise Enable/Disable, for SiFa and PZB (I bet your mini keyboard doesn't have PgDn, End and Delete keys), for enabling/disabling safety systems (Ctrl+Enter, Shift+Enter).
Unfortunately, there is no keyboard shortcut yet. This doesn’t work with the P'LINO 390, but it does with the German trains. Subject to change by Dovetail Games?!