How Can I Add A Derived Class To Repository

I need an advice. I have two class and one ofe them a derived class. I want to insert my base and derived class in a one method. How can i accomplish this?

//my entities
public class Form
{
    public int FormId { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public string Message { get; set; }
}

public class ReturnForm : Form
{
    public int OrderId { get; set; }
}

//my repository
public class FormRepository : IFormRepository:IDisposable
{
    private readonly MyContext _db;

    public FormRepository(MyContext db)
    {
        _db = db;
    }
    
    //I want to insert here Form or ReturnForm objects.
    //What can i do?
    public void AddFormMessage(Form form)
    {
        _db.Forms.Add(form);
    }
}