I am able to click the "Shapes" button but has stuck on the next step.
activate application "Preview"
tell application "System Events"
tell process "Preview"
click button 3 of toolbar 2 of front window
click UI element 1 of UI element 1 of pop over 1 of front window
I actually confuse by what you post. The image indicates a Screen Capture window not Preview app, but the Applescript references the Preview app. If it is the Preview app, then you can actually use the default shortcut key.