[Tiwaz]

Install the software that came with the mouse (maybe Logitech MouseWare works as well, but try the original software first).

In the mouse options, assign a key to that particular mouse button (e.g. 'backspace' or some character).

In MoHAA, assign that same key (maybe clicking that button will result in the same) to the function you want to use the button for.

Good chance this is the solution.