private void addTabToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (tabControlMain.SelectedTab.Tag == null)
                return;

            string tab = tabControlMain.SelectedTab.Tag.ToString();
            int idx = tabControlMain.SelectedIndex;
            string str = "";

            TabParams tab_param = new TabParams();

            if (tab == "11")
            { 
                param_tabs_count++;
                str = "PARAMETERS" + param_tabs_count.ToString();

                tab_param.Type = TAB_TYPE_PARAMS;
                tab_param.Index = param_tabs_count;
                tab_param.Tag = idx + param_tabs_count;

                AddParamTab(tabControlMain, str, idx + param_tabs_count);
            }
            else if (tab == "12")
            {
                script_tabs_count++;
                str = "SCRIPT" + script_tabs_count.ToString();

                tab_param.Type = TAB_TYPE_SCRIPT;
                tab_param.Index = script_tabs_count;
                tab_param.Tag = idx + script_tabs_count;

                AddScriptTab(tabControlMain, str, idx + script_tabs_count);
            }

            tab_param.Name = str;

            tabs.Add(tab_param);
        }

public partial class UserControlParams : UserControl
{
        public UserControlParams(SerialPort ser_port, ref List<TabParams> tab_list)
        {
            InitializeComponent();
            serial_port = ser_port;
            tablist = tab_list;
        }

      ///////////////////////////////////////////
}

private void buttonParamsLoad_Click(object sender, EventArgs e)
        {
            string file_path;

            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Filter = "XML Files (.xml)|*.xml";
            DialogResult result = ofd.ShowDialog();

            if (result == DialogResult.OK)
            {
                file_path = ofd.FileName;
                //textBoxFileName.Text = ofd.FileName;

                parameters = LoadParametersFromFile(file_path);
                LoadParametersToGrid(parameters);

                foreach (TabParams tab in tablist)
                {
                    if (tab.Tag.ToString() == this.Tag.ToString())
                        tab.File = file_path;
                }
            }
        }

private void AddParamTab(TabControl tab_contr, string tab_name, int idx)
        {
            TabPage tab_page = new TabPage();
            UserControlParams user_contr = new UserControlParams(serial_port, ref tabs);
            tab_contr.TabPages.Insert(idx, tab_page);
            tab_page.Text = tab_name;
            tab_page.Tag = "param";
            user_contr.Dock = DockStyle.Fill;
            tab_page.Controls.Add(user_contr);
        }

 user_contr.Tag = idx.ToString();

public class TabParams
    {
        public string Name { get; set; }
        public int Tag { get; set; }
        public int Index { get; set; }
        public int Type { get; set; }
        public string File { get; set; }
    }

//где то снаружи
public List<TabParams> tabs = new List<TabParams>();

TabParams tab_param = new TabParams();

//бла бла бла

tabs.Add(tab_param);

[XmlRoot("Settings")]
public class MySettings
{
    [XmlArrayItem("Tab")]
    public List<TabInfo> Tabs { get; } = new List<TabInfo>();
}

public class TabInfo
{
    public string Name { get; set; }
    public int Index { get; set; }
    public string File { get; set; }
}

public partial class frmMain : Form
    {
        private MySettings _Settings;
        private void LoadSettings() { }
        private void SaveSettings() { }
        //По нажатию кнопки на форме, добавляешь вкладку в контрол здесь
        private void AddTab(TabInfo tabInfo) { }
// ...