Save Foreign Key Value - EF Core + ASP.NET Core

Hi guys, I´m new to ASP.NET Core and I have the following problem:

I have two related objects, Goals and Months, Goals got the foreign key of Months, and when I try to save the data of goals I got a foreign key error because the value of Months is 0, but when I create the select in the HTML I got correct code value.

Description of the error:

SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Goals_Months_MonthId". The conflict occurred in database "Database Name", table "dbo.Months", column 'MonthId'.
The statement has been terminated.

Here´s the code:

HTML

@model ProjectName.Models.Goals
<div><form asp-controller="Goals" asp-action="Create">
      Other form elements<select name="months" asp-for="MonthId" class="form-control" asp-items="ViewBag.Months"></select><input type="submit" value="Save" class="btn btn-primary" /></form></div>

Goals Model

public class Goals
    {
        [Display(Name = "Code")]
        public Int32 GoalId { get; set; }

        public String GoalName{ get; set; }
        public Months Month{ get; set; }
        public Int32 MonthId{ get; set; }
        ....
        Other properties

    }

Month Model

    public class Months
    {
        [Key]
        [Display(Name = "Code")]
        public Int32 MonthId { get; set; }
        [Display(Name = "Designation")]
        public String Designation{ get; set; }
    }

Goals Controller

public IActionResult Create(Goals goal)
{
   if (ModelState.IsValid)
   {
      _context.Goals.Add(goal);
      _context.SaveChanges();
   }
   return View("~/../Create.cshtml");
}

Best Regards