how to Pass parameters from c# to oracle function

I am executing below function within my c# program

CREATE OR REPLACE FUNCTION F_INSERT_ORDER_DATA (  P_CUSTOMER_ID     IN NUMBER,
                                                  P_NOTES                  IN VARCHAR2,
                                                  P_CREATED_BY         IN VARCHAR2)
   RETURN NUMBER
IS
   vCREATED_ON   DATE := SYSDATE;
   vORDER_ID     NUMBER;
BEGIN
   INSERT INTO orders ( ORDER_ID,
                                        CUSTOMER_ID,
                                        NOTES,
                                        CREATED_BY,
                                        CREATED_ON)
                        VALUES ( NULL,                             --ORDER_ID Filled by trigger
                                        P_CUSTOMER_ID,         --CUSTOMER_ID
                                        P_NOTES,                    --NOTES
                                        P_CREATED_BY,             --CREATED_BY
                                        vCREATED_ON)              --CREATED_ON
RETURNING ORDER_ID INTO vORDER_ID;

   RETURN (vORDER_ID);

END;

but I am getting error that I cant insert null into CUSTOMER_ID column so the error is by passing parameters from c# to oracle function I need someone to help my by passing parameters ?

    private void B_Insert_Click(object sender, EventArgs e)
    {
        if (PaidCash < TotalInv) 
        {
            MessageBox.Show("paid amount less than cost", "be careful", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            TB_PAID_CASH.BackColor = Color.Tomato;
            TB_PAID_CASH.Focus();
            return;
        }
        else if (Reminder < 0)
        {
            MessageBox.Show("zero value", "be careful", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            TB_Discount.BackColor = Color.Tomato;
            TB_Discount.Focus();
            return; 
        }
        else
        {
            Form_control("DataInserted");

            string connstr = @"Data Source=orcl; User Id=admin; password=123123;";

            string insertcmdtxt = @"F_INSERT_ORDER_DATA";   //~ F_INSERT_ORDER_DATA ~//

            using (OracleConnection conn = new OracleConnection(connstr))
            using (OracleCommand cmd = new OracleCommand(insertcmdtxt, conn))
            {
                try
                {
                    conn.Open();

                    cmd.CommandType = CommandType.StoredProcedure;

                    cmd.CommandText = insertcmdtxt;

                    cmd.Parameters.Add(new OracleParameter("P_CUSTOMER_ID",TB_CUSTOMER_ID.Text ));
                    cmd.Parameters.Add(new OracleParameter("P_NOTES", null));
                    cmd.Parameters.Add(new OracleParameter("P_CREATED_BY", "System"));
                    cmd.Parameters.Add(":vORDER_ID", OracleDbType.Int64, ParameterDirection.ReturnValue);

                    cmd.ExecuteNonQuery();

                    TB_INVOICE_ID.Text = (cmd.Parameters[":vORDER_ID"].Value).ToString();

                }
                catch (Exception EX)
                {
                    MessageBox.Show(EX.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    return;
                }
            }
        }
    }