Mapping a field that doesn't exist in a table as one of composite Foreign Keys EF6

Question

I have two tables. Invoice and Plant.

 public partial class Invoice
{  
    [Key]
    public int InvoiceId{ get; set;}
    [Foreignkey("Site"] 
    public int SiteId{ get; set; }

    //There is no field in this table that gives me the corporateId    

    [InverseProperty("Invoices")]
    public Site Site{ get; set; }

}

     public partial class Sites
{  
    [Column(Order = 0)]
    [Key]
    public int SiteId{ get; set;}
    [Column(Order = 1)]  
    [Key] 
    public int CorporateId{ get; set; }

    [InverseProperty("Site")]
    public ICollection<Invoices> Invoices{ get; set; }

}

I want to use Context.Invoices.Include(s => s.Site) instead of a join. I know which CorportageId I need. I would like to set it the same for each invoices. I tried by adding a field in Invoices but EF doen't map it to the right key. Any idea?