If you want to use extensions with the Microsoft Edge browser, there are a few steps you need to take. First, you need to be on an Insider Preview build from the Redstone Development Branch. You also need to install them manually.
Of course, that won't always be the case. Microsoft has said that extensions will have a one-click install process from the Store at some point, likely when the Anniversary Update ships. Now, WindowsBlogItalia gives us our first look at what that will be like.
Once they're in the Store, you'll still be able to side-load extensions as you wish; however, there will be a "Not from the Store" message attached to it.
Installing via this method will also change the location where it is stored.
Extensions will be marked in the Store with the message "This product also contains an extension for Microsoft Edge". While that's not entirely surprising, the phrasing implies that there will be standalone extensions, as well as apps that come bundled with them.
Aside from OneNote Web Clipper, there are also a couple other extensions that are currently taking up residence in the Windows Store.
Since Store listings already exist, it wouldn't be surprising if Microsoft made the feature available to Insiders prior to the release of the Anniversary Update. After all, one-click installs are an important feature, as it's something that competitors have had for a long time.