First, the subwoofer has to be self-powered, or you will need to add an amplifier for the subwoofer.
The sub-out output is a full range line-out which means a low pass filter is required. Many subwoofers have low-pass filters built-in.
Connect the audio cable from the 3.5mm subwoofer output to the line input of the subwoofer. A line input usually comes in the form of 3.5mm, RCA or 1/4 photo/TRS connectors, NOT speaker level input such as bare wire clippers or banana connectors.
Our subwoofer T5 is designed to match all Edifier and Airpulse bookshelf speakers: