How to get settings from appsettings.json in a referenced .NET Framework project

Well... I didn't get the answer I would like to have. So I did a workaround and I had to change some code in the DAL project. Hopefully it didn't break some functions for other projects...

In DAL project:
Since I was working with a EF Entity with Model first, I couldn't change the model. But it created partial classes, so I did another partial class for MyEntity with a contstructor that takes a parameter.

namespace DAL.Models
{
    public partial class MyEntities
    {
        public MyEntities(string connString) : base(connString)
        {
        }
    }
}

And in the DAL repo I created a constructor with parameter (for injection from my .NET core project) and a private method.

public class UserRepository : IUserRepository
{
    private readonly string _dbContextString;

    public UserRepository(string dbContextString)
    {
        _dbContextString = dbContextString;
    }

    private MyEntities GetMyEntities()
    {
        return string.IsNullOrEmpty(_dbContextString) ? new MyEntities() : new MyEntities(_dbContextString);
    }
    ...
}

And then I did a search and replace for uses of MyEntity(), from

    using (var context = new MyEntities())
    {
        ...

to

    using (var context = GetMyEntities())
    {
        ...

In my .NET core project:
In Startup.cs I added a line in ConfigureService to inject the connectionString in my repo.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddSingleton<IUserRepository>(new UserRepository(Configuration.GetConnectionString("MyEntities")));
    ...

Tada, now it works with my new .NET core project and still working with the other .NET framework projects!