Add item to combobox dynamically

As in previously stated answers, dynamically inserted elements will disappear if the program restarts. This is simply because they are not recorded to a physical drive and will be removed as soon as the garbage collector realizes their work is done.
Simplest way to avoid this is to write contents of a combobox to a text file (if you do not want to meddle with databases) and save that file (as comboboxName.txt, for example). The second time program opens and shows form2, first add the items in the text document to the combobox.

/*write contents of a combobox to a file in a directory. the file location can be a predefined one or you may prompt the user for location by savefiledialog */
//this one saves to MyDocuments
//use on OnClosing event of the form - first delete existing file by system.IO [File.Delete - not in this code]
StringBuilder sb = new StringBuilder();
foreach(var item in ComboboxName.Items)
{
    sb.AppendLine(item.ToString());
}
string folderPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
using(StreamWriter _sw = new StreamWriter(folderPath+@"\ComboboxName.txt"))
{
    _sw.Write(sb.ToString());
}

//this one reads it from my documents and fills the combobox
//use on loading event of the form
string filePath=Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)+@"\ComboboxName.txt";
using (StreamReader sr = new StreamReader(filePath)
            {
                String line;
                while ((line = sr.ReadLine()) != null)
                {
                    ComboboxName.Items.Add(line);
                }
            }

One point is, if you try to save the file into sensitive folders, your program might need to be executed as elevated (i.e. run as administrator)
Of course these codes are almost pseudocodes. For better performance compare existing lines in the text file and combobox and write/delete only if necessary.