How to save data in a one to zero or one relationship with Entity Framework

I have two tables that have a relationship of one to zero or one, InvoiceSale and Budget. A sales invoice can have a Budget (or not), but every Budget belongs to exactly one sales invoice.

This is my InvoiceSale class:

public class InvoiceSale
{
    public int Id { get; set; }        
    public int NumberInvoice { get; set; }
    public DateTime InvoiceSaleDate { get; set; }
    public Budget Budget{ get; set; }
}

And this is my Budget class:

public class Budget
{
    public int Id { get; set; }
    public byte StateId { get; set; }
    public long Total { get; set; }
    public InvoiceSale InvoiceSale { get; set; }
}

And this is my fluent API configuration of the relationship:

HasOptional(fv => fv.Budget)
   .WithOptionalDependent(p => p.InvoiceSale)
   .Map(m => m.MapKey("BudgetId")); //I don't have a navigation property for 
    //this

When I send data from a form, I send the BudgetId value of a drop-down list, but when I try to save the changes in Entity Framework, the BudgetId which is an optional field always inserts null.

Since I do not have a navigation property for BudgetId, I don't know how to send this value to the database with Entity Framework.

Here is the controller action that inserts the data:

[HttpPost]
public ActionResult CreateInvoiceSale(InvoiceSale newInvoiceSale)
{
    var invoiceSale = new InvoiceSale
    {                    
        NumberInvoice = newInvoiceSale.NumberInvoice ,
        InvoiceSaleDate = newInvoiceSale.InvoiceSaleDate ,
        // Here I need to pass the BudgetId value, but how can I do it 
        // without a navigation property?
    };

    _context.InvoiceSales.Add(invoiceSale);
    _context.SaveChanges();

    return Json("ok"):
}