In the screen that you want to print you just need to:

1.  insert a button/link on the screen;
2. in the action associated with the button/link call the public action "PrintCurrentPage" of the PrinterPlugin.

To test you need to generate a native app and install it in a device, it won't work in the Web Preview or with OutSystems Now.

When you tap the button/link it will open the operating system's printer screen: