Short answer:
The following command sequence inserts a \medskip in your document, you just have to bind it to the shortcut you prefer: inset-insert vspace medskip.
General answer:
At times it is a bit tricky to figure out what LyX commands one actually has to bind to a shortcut to achieve the desired behavior. While for most commands LyX shows the last executed command in the status bar, this does not help for commands that open a dialog for further settings. In the following, I describe the approach I used to figure this one out, so you could apply it for other questions in this respect:
(1) Start Lyx with the -dbg action option (that is, run /path/to/lyx-binary/lyx -dbg action from a console window).
(2) Execute the command for which you want to figure out the command sequence. LyX lists the actions in the debug output in the console. The following shows the relevant output for Insert->Formatting->Vertical Space... with selecting MedSkip:
LyXFunc::dispatch: cmd: action: 219 [dialog-show-new-inset] arg: 'vspace' x: 0 y: 0
LyXFunc.cpp(771):
LyXFunc::dispatch: cmd: action: 218 [dialog-show] arg: 'vspace vspace defskip' x: 0 y: 0
LyXFunc.cpp(1842): dispatch msg is
LyXFunc.cpp(1875): verbose dispatch msg (dialog-show-new-inset vspace)
LyXFunc.cpp(771):
LyXFunc::dispatch: cmd: action: 224 [inset-apply] arg: 'vspace medskip' x: 0 y: 0
LyXFunc.cpp(771):
LyXFunc::dispatch: cmd: action: 225 [inset-insert] arg: 'vspace medskip' x: 0 y: 0
(3) Usually the last command executed by some dialog-based action is the one you are looking for. In this case it is the line with [inset-insert] and the arguments vspace medskip.
(4) Use the LyX Mini-Command Buffer (View -> Toolbars -> Command Buffer) to manually test the command: inset-insert vspace medskip
If the result is what you want to achieve, you are done and have found the command sequence to bind to the shortcut. Otherwise you have to experiment more :-)
Update: General answer using the GUI only
For those who do not feel comfortable with starting LyX from a console window, there is also a GUI-only way to figure out the commands (thanks to Dominik for the hint!):
(1a) In LyX, open the debug Console by selecting View –> Message Pane
(1b) Configure the settings on the Settings page, so that User commands and External control interface are set to Yes:
(2) Switch on the Output pane and execute the command for which you want to figure out the command sequence. The following shows the relevant output for Insert->Formatting->Vertical Space... with selecting DefSkip:
(3) Proceed as above.
Go to the menubar and press
Tools | Preferences | Editing | Shortcuts
In the window that has popped up, enter newpage in the field called Show key-binding containing. In the window below, you will see a function called
newpage-insert
Highlight this and press Modify. In the window that pops up, you can enter your favorite shortcut (I have used Alt+Shift+N).
Press Apply first and then Save, and you should be done.
Works for me under Linux with LyX 2.0.4.
Best Answer
It might depend on your OS, but in general: holding the Alt key will show you the shortcut key. (press and hold Alt) - i - o - n - (release Alt) will start a new page. Cross-reference with Alt - i - r.