Usually there's a menu item corresponding to the key combination and the simplest thing is to just use that. Record yourself selecting the item from the menu and you'll see what's required.
If there isn't a corresponding menu item, try recording yourself pressing the key combination and see what you get. I mention this second because often the browser will intercept function key combinations and not let them through to Personas to be recorded. There is a "Press F key" action in Personas scripting, but to be honest I've never used it and so I'm not sure how to specify "Ctrl-Shift-F1", for example.
Steve.