How to create an instance of UserManager in ASP.NET MVC Core 2

I did not have access to DI.

this explanation is incorrect, you can inject UserManager in another assembly read more.

Just create a service for seeding your data for example:

public interface IInitializationService
{
    void Seed();
}

public class InitializationService : IInitializationService
{
    private readonly UserManager<ApplicationUser> _userManager;

    public InitializationService(UserManager<ApplicationUser> userManager)
    {
        _userManager = userManager;
    }

    public void Seed()
    {
        // more code
    }
}

Register service in Startup.cs

 public void ConfigureServices(IServiceCollection services)
 {
        services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlite(Configuration.GetConnectionString("DefaultConnection")));

        services.AddIdentity<ApplicationUser, IdentityRole>()
            .AddEntityFrameworkStores<ApplicationDbContext>()
            .AddDefaultTokenProviders();

        services.AddTransient<IInitializationService, InitializationService>();

        services.AddMvc();
 }

 public void Configure(IApplicationBuilder app, IHostingEnvironment env)
 {
        // more code ...

        var scopeFactory = app.ApplicationServices.GetRequiredService<IServiceScopeFactory>();
        using (var scope = scopeFactory.CreateScope())
        {
            var identityDbInitialize = scope.ServiceProvider.GetService<IInitializationService>();
            identityDbInitialize.Seed();
        }

      // more code ...
 }