How does an external system authenticate with web api plugin in admin area?

I needed a very simple web api for an external system to get customer data from nopcommerce. I took this plugin (noprest) and made it work with nopcommerce 4.3 and it works great. But when making the call to the web api url, it requires authentication because it is in the admin area. How would an external system that needs to call this api authenticate? https://archive.codeplex.com/?p=noprest
Thanks for any advice!