Form1: Add combobox item from another form2: textbox Pin

You can use singleton pattern to solve the problem as below

Modify your code in Form1 as below

public partial class Form1 : Form
{
    static Form1 _instance;
    public static Form1 Instance
    {
        get
        {
            if (_instance == null)
                _instance = new Form1();
            return _instance;
        }
    }

    public void AddItemToCombobox(object value)
    {
        comboBox1.Items.Add(value);
    }

    public Form2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        using (Form2 frm = new Form2())
        {
            frm.ShowDialog();
        }
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        _instance = this;
    }

Open your Form2 then code to OK button as below

private void button1_Click(object sender, EventArgs e)
{
    Form2.Instance.AddItemToCombobox(textBox1.Text);
    textBox1.Clear();
}

TextBox, Button in Form2 i'm using default name, similar Form1

I hope you can solve your problem