Track whether a user typed an specific "word" on an WinForm

I've a ScreenLocker winform application. It's full-screen and transparent. It unlocks the screen when user presses Ctrl+Alt+Shift+P.

But I want it more dynamic. I would like to let a user set his own password in a config file.

For example, he set his password mypass. My problem is- how can I track whether he typed 'mypass' on that form?

Help please.

Here is my current code-

    public frmMain()
    {
        InitializeComponent();
        this.KeyPreview = true;

        this.WindowState = FormWindowState.Normal;
        this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
        this.Bounds = Screen.PrimaryScreen.Bounds;
        this.ShowInTaskbar = false;
        double OpacityValue = 4.0 / 100;
        this.Opacity = OpacityValue;
    }

    private void OnKeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.P && e.Modifiers == (Keys.Control | Keys.Shift | Keys.Alt))
        {
            this.Close();
        }
    }