How do I add scrollbars to C# forms during runtime...?

It depends on what control or controls you want to have the scroll bars.
Bear in mind that the Form is also a Control, so if you want to add scroll bars to that, you can do it in exactly the same way as any other container: by setting the AutoScroll property[^] to true.
Then, if any controls are outside the current form display area, the appropriate scroll bars will automatically appear.