Create role using entity framework core migrations

I'm creating my 4th migrations script with EF Core (2.0.0). In there I want to add a few roles to the database.

The problem is, is that I'm not really sure how to do this. Currently I have this:

protected override void Up(MigrationBuilder migrationBuilder)
{
    // todo: Pass connection string somehow..?
    var opt = new DbContextOptions<ApplicationContext>();
    var roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(new ApplicationContext(opt)));

    //if (!roleManager.RoleExists("ROLE NAME"))
    //{
        // todo: create the role...
    //}
}

But creating the RoleManager like that gives me the following error:

There is no argument given that corresponds to the required formal parameter 'roleValidators' of 'RoleManager.RoleManager(IRoleStore, IEnumerable>, ILookupNormalizer, IdentityErrorDescriber, ILogger>)'

I'm not sure how to solve this problem. I couldn't find any info on how to do this properly in .NET Core using migrations.

I'm facing two issues in this code:

• I'm trying to create an instance of the DbContext somehow. Shouldn't I be able to get the DbContext from within my migrations code?
• Instantiating the RoleManager like this doesn't work and needs to be resolved.

How can I solve these problems?