How to stop an action

I'm trying to learn C# by creating a simple Windows Form Application.

I have a start button which starts a process inside a while loop when clicked.

I also have a stop button and I would want it to stop the while loop initiated when my start button was clicked.
Problem is, it does not work.
I also have another mechanism which is when I press the Esc button it stops the while loop. This mechanism however works fine.
This is my code:

class Global
{
    public static string PrcStpr = "off";
    public static string KeyVal;
}

public Form1()
{
    InitializeComponent();
    this.KeyPreview = true;
    this.KeyDown += Form1_KeyPress;
}

private void Form1_KeyPress(object sender, KeyEventArgs e)
{
    Global.KeyVal = e.KeyCode.ToString();
}

private void button2_Click(object sender, EventArgs e) //this is my stop button                                                          
{
    Global.PrcStpr = "on";
}

private void button1_Click(object sender, EventArgs e) //this is my start button
{
    Global.KeyVal = "";
    Global.PrcStpr = "off";
    while (Global.KeyVal != "Escape" || Global.PrcStpr != "on")
    {
        richTextBox1.Focus();
        SendKeys.SendWait(" ");
    }
}

Can anyone help me with this please?
What am I doing wrong?

It is important to note that there is only one UI thread. This means that if the UI thread is doing any kind of processing it cannot do other things, like updating the UI or processing button presses etc.

Your current code does not make much sense to me. I would guess that it will just put a bunch of messages on the message queue, but never get a chance to process them, effectively hanging the application.

So if you need to do something time consuming there are a few possible approaches:

1. Asynchronous processing- This is appropriate if you are doing IO operations, like reading from disk, a database, or marking some webrequest.
2. Background processing - Move the execution to a background thread. This is suitable for compute heavy operations. This should use Tasks to run the code and CancellationToken for cancelling.
3. Timer - Create a timer that runs some piece of code at regular (or non regular) intervals. Timers can run the code either on a background thread or UI thread.

My guess is that you want a timer, with an interval of, say 100ms? Put the contents of your loop in an event handler for the Tick-event, and make your buttons set the Enable property to true/false.

But it is not at all clear what it is you are actually trying to do. Repeatedly setting focus, or using Send Keys like this, seem odd to me. So there is a significant risk that this is a XY-problem.