How to add a scroll to bootbox dialog?

Bootbox uses a css class named modal-body for the body of its dialog; so you can override the class to make it scroll-able. For instance:

.modal-body {
    height: 100px;
    overflow-y: scroll;
}

If you want your dialog to expand and add scrollbars based on the size of your content, use this:

.modal-body { 
   max-height: 500px; 
   overflow-y: auto; 
}

That sets the maximum height, after which the scrollbar is added. So if you have little content, no bar and height adjusts to fix. If you have lots of content, you get a scrollbar and the height doesn't get too crazy high.