Add events to custom user controls

Create an event, for example checked changed on a CheckBox.

public partial class UserControl1 : UserControl
{
    public delegate void OnCheckChanged(CheckBox sender);
    public event OnCheckChanged OnCheckChangedEvent;
    public UserControl1()
    {
        InitializeComponent();
    }

    private void checkBox1_CheckedChanged(object sender, EventArgs e)
    {
        OnCheckChangedEvent?.Invoke((CheckBox)sender);
    }
}

Subscribe in the form to the event above

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        DemoControl.OnCheckChangedEvent += DemoControlCheckChanged;
    }

    private void DemoControlCheckChanged(CheckBox sender)
    {
        MessageBox.Show($"{sender.Checked}");
    }
}

Or change the Modifiers property of the control, in this case a CheckBox from private to public than directly subscribe to the event. This is not recommended, always scope to the smallest needs possible.