Microsoft Edge browser which has got extensions support with anniversary update, not only lets you to install extensions by visiting Windows Store, but also allows to install extensions from outside Store also. For that, you need to visit about:flags and check the ‘Enable extension developer features’ option. After that, you can able to load the extension by selecting the folder you’ve extracted from a zip file, developer has offered on their website.
Edge keeps turning off extensions from unknown sources
There is one reason why you shouldn’t install an extension that is not available on Windows Store as an end user in Edge browser: it keeps disabling the extension and alerts you about the same that ‘We’ve turned off extensions from unknown sources. They might be risky so we recommend keeping them off.’, and offers ‘Turn on anyway’ and ‘Keep them off’ options.
Chrome also does this when you install an extension using developer mode, it continuously bugs to disable the extension on each start of browser.
It makes sense for a developer to test his unsigned extension that was by side-loading, but not for end user, he either needs to find alternate extension on the store or wait for the developer to release the extension in question.