To create games for Windows 8 you will need to have a copy installed on your PC. This can be the main OS or a sub-os that you can dual boot with Windows 7 (or any other OS you choose to use) but you cannot create a final application for Windows 8 outside of that OS.
When you have Windows 8 installed, you must make sure that you have Microsoft .Net Framework 3.5 installed (.Net 4 is installed as standard with Windows 8, but you may need to install the previous version too) which is available here, although when you install GameMaker:Studio on your Windows 8 OS it should complete this part automatically.
Microsoft Developer Licence
The GameMaker:Studio Windows 8 export target will create an app specifically for the Windows 8 OS, and it will be compatible with many of the unique features that this OS offers the user. However, if you wish to publish this game through the Microsoft Store for Windows, you must have a Microsoft Developers Licence which is available by signing up for an account here. There is a small cost involved in registering for this account and you will need to have a valid bank account and credit facilities to be able to monetise any apps that you submit to the store.
Once all that is done and you have GameMaker:Studio installed, it is worthwhile taking a moment to look over the Microsoft Windows 8 Developers Guide as it contains a vast amount of information and recommendations for creating your apps and games so that they are integrated properly into this new and dynamic OS.
Once you have completed the preparation detailed above, it is a simple thing to start up GameMaker:Studio and begin to create your apps and games for Windows 8. There are a number of new functions related to this OS (you can find them easily as they are all suffixed with win8_) which will enable you to use the Search, Live Tile and Sharing features but before diving in it is worthwhile setting up your Developers Key correctly.
The Developer Key is essential to creating a Windows 8 app but it cannot be generated until you have set up the Preferences correctly in GameMaker:Studio.
However, if these values are incorrect or are not generated automatically for you, you should do the following:
Finally you should click on the Check Windows 8 SDK button to confirm that your selections are okay and that everything works as it should. If you get an error or you are not sure that your Developer Licence is correct, you may also click on the button labelled Acquire Developer Licence which will open up a command prompt and automatically go through the necessary steps to get you one.