You have two options;
create the enumeration list yourself
download the module in the latest 6 modeler, covert to 7, to latest version 7, covert to 8.
Decide which is the least amount of effort for you
Open the AppStore, search for Country Enumeration and download it to your app.
There are a couple of modules in the app store you can try. Here is one of them:
Just create a new 7.23 project, download the module and export the enum. That file you can use in Mx8. Will see if I can make some time free to create the Mx8 version.