private void Add_btn_Click(object sender, EventArgs e)
        {
            // get values from text boxes if they were inserted by user
            string   username    = username_textbox.ForeColor == Color.Black ? username_textbox.Text : "";
            string   email       = email_textbox.ForeColor == Color.Black ? email_textbox.Text : "";
            string   fname       = firstname_textbox.ForeColor == Color.Black ? firstname_textbox.Text : "";
            string   mname       = middlename_textbox.ForeColor == Color.Black ? middlename_textbox.Text : "";
            string   lname       = lastname_textbox.ForeColor == Color.Black ? lastname_textbox.Text : "";
            string   cardNo      = cardnumber_textbox.ForeColor == Color.Black ? cardnumber_textbox.Text : "";
            int      periodicity = DB_API.SelectRecurenceIdbyDesignation((string)Periodicity_comboBox.SelectedItem);
            DateTime term        = DateTime.Parse(term_dateTimePicker.Value.ToString());

            // verify if email field is filled
            if (email.Equals(""))
            {
                ErrorMessenger.EmptyField("Email");
                return;
            }

            // verify that all mandatory fields are filled (if subscripition is checked)
            if (active_checkBox.Checked)
            {
                if ("".Equals(fname) || "".Equals(lname) || "".Equals(cardNo) || "".Equals(periodicity))
                {
                    ErrorMessenger.EmptyField("Every field marked (*)");
                    return;
                }
            }

            // verify if user exists, if so, cannot be added, only updated
            var exists = DB_API.ExistsUser(email);

            if (exists)
            {
                ErrorMessenger.Error("User already exists!");
                return;
            }

            // add user
            DB_API.InsertUser(username, email, fname, mname, lname, cardNo, periodicity, term, active_checkBox.Checked);

            // upadte listBox with new user
            PopulateUsersListView();
        }