We’ll do it the simplest possible way – the interface would be the well-known IExtension and we’ll inherit from ExtensionBase redefining the Name:

using CoreXF.Abstractions;

namespace Extension1
{
    public class TheExtension : ExtensionBase
    {
        public override string Name => "Some name";
    }
}

That’s all – the simplest possible extension, inheriting from ExtensionBase and overriding one of its properties.

The IExtensionRegistry is used to retrieve the required extension:

public IActionResult Index()
{
    var extension = this.registry.GetExtension<TheExtension>();
    this.ViewBag.ExtName = extension?.Name;
    return this.View();
}

A realistic scenario

Of course, in a more realistic scenario, the interface should be put in a separate assembly and referenced by the host application. The extension then can be refactored as many times as needed in order to achieve the required functionality.