PC's that come with Windows 8 preinstalled at the factory have an embedded key. Official Windows 8 install media will auto detect it and install without asking you to input a key. The version that it originally shipped with will be installed. My ASUS laptop came with windows 8 core. When I do a clean install that is what is installed automatically because that is the version that the embedded key is for. To go to 8 Pro I have to enter the Pro key though add features after 8 core is installed. I can then also add Media Center by entering the MC key though the add features option. You don't need to know what your key is as you will never be asked to enter it. It is entered automatically during the install. That's why there is no COA sticker, its not needed. If your PC shipped with * Pro that's what will be installed.
If you are just upgrading from 8 to 8 Pro just enter your 8 Pro key though the add features option.