This shows that macOS is capable of generating middle mouse click events, it's just that VirtualBox does not by default create any useful mapping, such as Left Option + Click, to allow users to middle click from a basic macOS environment.
You need to get a better mouse, a mouse that has an actual middle-click mechanism (3 or more buttons). Whatever program you use, it must be setup to understand that simultaneous left+right=middle, it doesn't happen automagically. I tried attaching attaching an external mouse and pressing Left and Right click simultaneously, no luck.
