Setting up a button on your Quick Keys or a button on your pen to perform a screen capture takes a few easy steps. 

First, you will want to connect your Quick Keys and your Pen Tablet to your Mac.

Once you open the Settings program, you will see your devices in the lower Device section.

Select the Quick Keys from the left-hand navigation bar.

Once you have selected the Quick Keys, you will need to choose the button you want to assign to perform the Screen Capture when it is pressed.

In this case, I have selected button #3.

Click the button [2] and select [3] Keystrokes from the pop-up menu.

In the Define Keystrokes window, you will want to click in the field [4] Keystrokes Keys and then click [5] Clear what is there.

Then you will want to click on the dropdown menu for [6] Special Functions. 

Once the menu is open, you have two selections you can make.

[7] Capture Screen will capture a portion of your screen area, and Copy Screen will allow you to capture your entire screen.

Make your selection. 

[8] You will see your selection listed in the Keystroke Keys field.

[9] Click OK to save your selection.

[10] You can not see your new setting applied to button #3.

[11] you will notice that I assigned this action to the "All Other" icon in the top Application row.

If you are making a screen capture in a specific application, you may want to add it to the Application row.

The instructions to do that are listed in the below FAQ and are the same whether you are using a Pen Tablet or the Quick Keys.