Difference between bake and roast
Any recipe which calls for you to bake, roast, or otherwise cook at a particular temperature, is expecting you to preheat the cooker to the specified temperature before adding the food, unless it specifically says otherwise.

"Roast" or "bake" never mean "microwave". If a recipe wants you to use the microwave, it'll say that. (There are, of course, things you can choose to do in a microwave or in some other cooker, like melting butter; that's up to you.)