This is an explanation on how to assign the Zoom function to the wheel of your Wacom tablet or to the button of your Wacom pen.
Please refer to the respective images below.
If your Wacom tablet has a wheel, you can assign it the mouse scroll-button function to zoom in and out. You can zoom by scrolling the wheel circle.
▼ Pen Button
If your Wacom tablet doesn't have a wheel, try assigning the zoom function to the pen button. You can zoom in and out by pressing the pen button and moving the pen up or down on the tablet.