Allow + and other non alphanumeric characters in UserName using ASP.NET Core 1.0, Identity 3.0 and Entity Framework Core 1.0

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
  // (Rest of code removed)

  // Note the + added to the string of allowed user name characters
  services.AddIdentity<ApplicationUser, IdentityRole>(o => o.User.AllowedUserNameCharacters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 -._@+")
    .AddEntityFrameworkStores<ApplicationDbContext>()
    .AddDefaultTokenProviders();
}