What is the key combination to move an application to a different workspace in Lubuntu?

Setting the shortcut

The command

To move an application (-window) to another workspace from command, you need wmctrl to be installed. The command you need to move the frontmost application (-'s window) to a specific workspace (on Lubuntu) is:

wmctrl -r :ACTIVE: -t 1

where 1 refers to the targeted workspace. The first workspace is 0, so the command above will move the active window to the second workspace.

Shortcuts in Lubuntu

In Lubuntu, to set custom shortcuts, (indeed) you have to manually edit the file ~/.config/openbox/lubuntu-rc.xml.
The easiest way to get the right syntax is to move to the keyboard section of the file, and simply copy an existing shortcut section, like the example below:

   <!-- move window to workspace 2 on Ctrl + Alt + Y-->
    <keybind key="C-A-Y">
      <action name="Execute">
        <command>wmctrl -r :ACTIVE: -t 1</command>
      </action>
    </keybind>

As a test, I added the section, and it worked fine (after log out/in); it moved the active window to the corresponding workspace on the shortcut Control+Alt+Y.

What you should keep in mind:

• The command needs wmctrl to be installed:

  sudo apt-get install wmctrl
  

• It takes a log out/in to start working

• The first workspace has index 0, so the command in the example actually moves the active window to the second workspace.

A more detailed instruction on adding custom shortcut keys and the used syntax, see here, in the section further down; Create New Keyboard shortcuts Also here, you can find a.o. how to "mention" the keys in your shortcut section:

Key combination

KEY-COMBINATION is the key combination which you want to bind to the actions. The format for the KEY-COMBINATION is: Modifier-Modifier-Key. Any number of modifiers (0 or more) can be used together, and they are each separated by a "-". Multiple KEY-COMBINATION can be specified in one keybind by separating them with spaces. This is useful if you use multiple keybinds for a thing that is quite complicated to specify.
Modifiers
Modifier keys   
S   Shift key
C   Control key
A   Alt key
W   Super key (Usually bound to the Windows key on keyboards which have one)
M   Meta key
H   Hyper key (If it is bound to something)

You can also use Mod1 through Mod5 as the modifiers to directly select a modifier mask (e.g. Mod1 to select Mod1Mask), but this is for advanced users who have customized their modifier mapping with xmodmap. 

More "sophisticated" options

You could create a script to see what is the current workspace, and move the active window to the next/previous one, instead of having to set a key combination for every (targeted) workspace.

That would however be another question :)