program.cs

class Program
{

    static void Main(string[] args)
    {
        var tenant1 = Guid.NewGuid();
        var tenant2 = Guid.NewGuid();
        int customerId = 1;

        Seed(tenant1, customerId, "Derek Comartin");
        Seed(tenant2, customerId, "CodeOpinion.com");

        using (var db = DbFactory(tenant1))
        {
            var customer = db.Customers.Single(x => x.CustomerId == customerId);
            Console.WriteLine($"Hello {customer.Name}");
        }

        using (var db = DbFactory(tenant2))
        {
            var customer = db.Customers.Single(x => x.CustomerId == customerId);
            Console.WriteLine($"Hello {customer.Name}");
        }

        Console.ReadKey();
    }

    private static MyDbContext DbFactory(Guid tenantId)
    {
        var connection = @"Server=(localdb)\mssqllocaldb;Database=EFCoreMultiTenant;Trusted_Connection=True;";
        return new MyDbContext(tenantId, connection);
    }

    static void Seed(Guid tenantId, int customerId, string name)
    {
        using (var db = DbFactory(tenantId))
        {
            db.Customers.Add(new Customer
            {
                CustomerId = customerId,
                TenantId = tenantId,
                Name = name
            });
            db.SaveChanges();
        }
    }
}