Have you tried Ctrl+N? Seems like an easier way to do it to me. Anyway, make a link to Opera where you want it and add --new-window after the last quotation mark in the target field of the shortcut. Make sure there is a space between the last quotation mark and the first dash. That is a Chromium command line option btw. Most of them should work in Opera since they share the same base. So any other Chromium command line options you know should just plug right in.