To be able to change the view location, you need to implement the IViewLocationExpander. The ExpandViewLocations method returns an IEnumerable<string> containing the different locations where to search, with

public class MyViewLocationExpander : IViewLocationExpander
{
    public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
    {
        yield return "/CustomViewFolder/{1}/{0}.cshtml";
        yield return "/SharedFolder/{0}.cshtml";
    }

    public void PopulateValues(ViewLocationExpanderContext context)
    {            
    }
}

You now need to register the Expander, in order for it to be used by the Razor View Engine. Just add this in the ConfigureServices of your Startup class.

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<RazorViewEngineOptions>(options => {
        options.ViewLocationExpanders.Add(new MyViewLocationExpander());
    });
}