This is mapped to the middle mouse button but there is no option to key map it. If you have a PC, you may be aware of key mapping mouse buttons so this invalidates the game mapping. This is common so it would be normal to remap the middle mouse button in game. However, I cannot see that option in your game. If I, or any person with a gaming mouse, wishes to change the hard mouse button map they will have to switch mouse profiles or mice. This is annoying.