public class MyDbContext
{
    public MyDbContext() : base(Connection(), true)
    {
        this.Configuration.AutoDetectChangesEnabled = true;
    }



    public DbSet<UserRepository.User> Users { get; set; }
    public DbSet<UserRepository.Group> Groups { get; set; }

    private static string ConnectionString = null;
    private static System.Data.Common.DbConnection Connection()
    {
        var con = System.Data.Common.DbProviderFactories.GetFactory("Npgsql").CreateConnection();
        con.ConnectionString = ConnectionString;
        return con;
    }
    public static bool RegisterProvider(string name, string desc, string invariant, string type)
    {
        try
        {
            var ds = System.Configuration.ConfigurationManager.GetSection("system.data") as DataSet;
            if (!ds.Tables[0].Rows.Cast<DataRow>().Any((row) => row["InvariantName"].ToString() == invariant))
            {
                ds.Tables[0].Rows.Add(name, desc, invariant, type);
            }
            return true;
        }
        catch
        {
            return false;
        }
    }

    public static void RegisterProvider()
    {
        RegisterProvider("Npgsql Data Provider", ".Net Framework Data Provider for Postgresql", "Npgsql", "Npgsql.NpgsqlFactory, Npgsql");
    }
    public static void SetConnectionString(string host = "localhost", string username = "postgres", string password = "", string database = "postgres")
    {
        ConnectionString = new Npgsql.NpgsqlConnectionStringBuilder
        {
            Username = username,
            Password = password,
            Host = host,
            Database = database
        }.ToString();
    }
}

Enable-Migrations -ProjectName "Project.MyDbContext"
Checking if the context targets an existing database...
System.ArgumentException: Unable to find the requested .Net Framework Data Provider.  It may not be installed.
   at System.Data.Common.DbProviderFactories.GetFactory(String providerInvariantName)
   at Project.MyDbContext.Connection() in Z:...MyDbContext.cs:line 33
   at Project.MyDbContext..ctor() in Z:...MyDbContext.cs:line 16
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Data.Entity.Infrastructure.DbContextInfo.CreateInstance()
   at System.Data.Entity.Infrastructure.DbContextInfo..ctor(Type contextType, DbProviderInfo modelProviderInfo, AppConfig config, DbConnectionInfo connectionInfo, Func`1 resolver)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, Boolean calledByCreateDatabase)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
   at System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldInitialCreate(String language, String rootNamespace)
   at System.Data.Entity.Migrations.EnableMigrationsCommand.<>c__DisplayClass2.<.ctor>b__0()
   at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
Unable to find the requested .Net Framework Data Provider.  It may not be installed.