Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using ApiModel.Data;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Identity;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace ApiModel
{
    public class Program
    {
        public static async Task Main(string[] args)
        {   
            
           var host = CreateHostBuilder(args).Build();

            //Run all migrations and update the db

            using (var serviceScope = host.Services.CreateScope())
            {
                var dbContext = serviceScope.ServiceProvider.GetRequiredService<DataContext>();

                await dbContext.Database.MigrateAsync();

                var roleManager = serviceScope.ServiceProvider.GetRequiredService<RoleManager<IdentityRole>>();

                if (!await roleManager.RoleExistsAsync("Admin"))
                {
                    var adminRole = new IdentityRole("Admin");
                    await roleManager.CreateAsync(adminRole);

                }

                if (!await roleManager.RoleExistsAsync("Poster"))
                {
                    var posterRole = new IdentityRole("Poster");    
                    await roleManager.CreateAsync(posterRole);

                }

            }

            await host.RunAsync();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });
    }
}