Unfortunately, it's not that simple, at least with the normal messagebox API (which I assume Purebasic uses on some level). The long and the short of it is, you can't actually alter anything about the messagebox short of what you can via the flags in the argument -Icon and buttons-. To change the box yourself you actually need to use windows hooks/subclassing, which is quite involved for a pretty simple request.
In my opinion it's not worth it, you'd be better off making your own customised box.