How to define

public event EventHandler LoginClicked;

How to call

LoginClicked?.Invoke(this, EventArgs.Empty);

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How to listen for it

INFO

This requires you to create an instance of the Custom Control in your mainWindow.xaml, with the x:name property!

<LoginRegisterLibrary:LoginControl x:Name="loginControl" />

public MainWindow()
{
    InitializeComponent();

    loginControl.LoginClicked += LoginControl_LoginClicked;
}

private void LoginControl_LoginClicked(object sender, EventArgs e)
{
	//HERE YOU CAN DO WHAT YOU WANT. Here is an example how to get some elements of the Custom Control here.
    string usernameOrEmail = loginControl.UsernameOrEmail;
    string password = loginControl.Password;

    MessageBox.Show($"Logging in with {usernameOrEmail} and Password {password}");
}