This file will allow you to pass a connection string as a parameter in powershell

Startupupdate.cs

using Microsoft.Data.Entity;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace YourDbContextNameSpace
{
    public class Startupupdate
    {
        public Startupupdate()
        {
            // Set up configuration sources.
            var builder = new ConfigurationBuilder()
                .AddEnvironmentVariables();
            Configuration = builder.Build();
        }
        public IConfigurationRoot Configuration { get; set; }
        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            var connection = Configuration["UpdateConnectionString"];
            // Add framework services.
            services.AddEntityFramework()
                .AddSqlServer()
                .AddDbContext<YourDbContext>(options => options.UseSqlServer(connection));
        }
    }
}