Doubt about insert in bank with foreign key using linq

I have a question, how to insert data into a mysql table, using linq, I have the tables below:

I'm using the method below to insert:

public void inserirCarteira(float cpf, string codigo, double valor, int quant, double vtotal)
    {
        try
        {

            validarInserir(cpf, codigo, valor, quant, vtotal);
            // instancia  o banco de dados
            bancotccEntities1 bco = new bancotccEntities1();
            bco.Database.Connection.Open();
            // cria um objeto 
            carteira cart = new carteira();
            // popula o objeto
            cart.cpf = cpf;
            cart.codigo = codigo;
            cart.valoracao = valor;
            cart.qtdacao = quant;
            cart.vtotalacao = vtotal;

            // adciona um objeto ao banco
            bco.AddTocarteira(cart);
            //salva o objeto no banco
            bco.SaveChanges();
            bco.Database.Connection.Close(); 
        }
        catch (Exception e1)
        {

            throw new Exception(e1.Message.ToString());
        }
    }

I know that I have to use my foreign key but I do not know what it would look like: cart.card_card ?

If this is how I am going to enter values since the idusuario is the primary key of the user table and is auto increment, then I do not know what the value of this field will be at the time of insertion. >

Can anyone give me an idea how to do it?