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

Hi guys, I´m new to ASP.NET Core and EF 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 MonthId 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_MonthId;. The conflict occurred in database Portal_RNT, table dbo.Months , column MonthId;.
The statement has been terminated.


Here is the code:

HTML

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

Goals Model

publicclass Goals
    {
        [Display(Name = "Code")]
        publicInt32 GoalId { get; set; }
 
        publicString GoalName{ get; set; }
        public Months Month{ get; set; }
        publicInt32 MonthId{ get; set; }
        ....
        Other properties
 
    }

Month Model

publicclass Months
{
    [Key]
    [Display(Name = "Code")]
    publicInt32 MonthId { get; set; }
    [Display(Name = "Designation")]
    publicString 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