Creating objects in ASP.NET Core 6 with EF Core, when they contain foreign keys

Question

I'm a beginner with .NET and I'm having a hard time understanding how to create objects with foreign keys.

Let's say I have this Localitate entity (which vaguely translates to city) which contains a FK to Judet (county):

namespace xx.DataLayer.Entities
{
    public class Localitate : BaseEntity
    {
        [Required]
        public string Nume { get; set; }
        [Required]
        public int JudetID { get; set; }
        [Required]
        public virtual Judet Judet { get; set; }

        public Localitate() { }
    }
}

Here's how my request looks:

public class LocalitateRequest
{
    public string Nume { get; set; }
    public int JudetID { get; set; }
}

When I create a Localitate object, how should I do it? Do I need to fill the navigation property too, or is the int JudetID enough?

If it is enough, I'm encountering another problem - if I insert a JudetID that doesn't exist, I don't get any error (and I think I should, my DB would do that if I would be doing an insert statement).