You don't need to add anything to your shortcut...
1. Do a search for the file "preyconfig.cfg" (without the quotes...)
(Should be in the Prey Demo\base folder in Program Files...)
2. Open this file in notepad, and scroll down or do a search to find this section:
seta r_customHeight "900"
seta r_customWidth "1400"
seta r_fullscreen "1"
seta r_displayRefresh "0"
seta r_mode "-1"
In your case set it exactly as I have here, and you will have the resolution you wanted... For anybody else, leave all as it is but change the "customheight" and "customwidth" to the values you require.
3. If you are using a widescreen 16:9 resolution you also need to set:
seta r_aspectRatio "1"
Otherwise 16:10 is "3" and 4:3 is "0"
Now Save and play!
Hope this helps!