I wish there was a simple way to just "Install" a high-score. But there are several things you need for a high-score table.
You need somewhere online to store the scores, a database that you can talk to. My company has developed a plugin to use with Parse.com, you can buy the plugin at our homepage: https://www.frostyelk.se/products-page/ ... /parse-fe/
You can also use Facebook for it, but it is rather complicated to get everything working with facebook, especially if you want to have the game working on Android and iOS.
If you do not plan on making a phone app, just an online game, then there are several options for ya. Do a search under tutorials: https://www.scirra.com/tutorials/search?q=high%20score