Under the General Tab, select "New action". Under "action type" under the drop down list select "keyboard shortcut->Command/URL (simple)"
Under the next tab choose "keyboard shortcut" and press the desired key, for example the "play" button on your keyboard. (If nothing happens you have to set your keyboard model under System settings--->keyboard). As you can see below my keyboard send "XF86AudioPause", the symbol on the actual key is a Play/pause symbol.
Then go to the next tab and set which command to send when the key is pressed, in our example it would make sense with "remoot play".