Breaking a loop when a button is clicked?

Question

C++ / C#

Hello, I am writing applications at the request of my teacher.
First, in the main form, on a button click, I call another second form, where I start an infinite loop (while (true)). This suits the teacher, since he really has to work all the time (cycle). Now I was told that from the first form I would stop the cycle by pressing the button to change the values ​​​​(parameters in the cycle).
Tell me how to do it? What needs to be rewritten and changed by the fact that you have not had to work with it before?
Thank you in advance for your response.