Diagnosis
You're looking for a physical button, but the Bluetooth pairing is activated from the remote.
How to Fix It
Grab the soundbar remote. Press and hold the 'Bluetooth' button (it has the Bluetooth symbol on it) for about three to five seconds. You'll see the front LED display start flashing 'BT' in blue. Now, on your phone, go to Bluetooth settings and look for "Polk Signa S2" in the list of available devices. Select it to pair. Once connected, the soundbar will show a solid blue 'BT' light.
If That Didn't Work
If the soundbar doesn't show up on your phone, it might already be paired to another device. From the soundbar remote, press the 'Source' button until you cycle to 'BT'. If it connects to an old device automatically, you need to clear that pairing first. With the soundbar in BT mode, press and hold the 'Bluetooth' button again for about 10 seconds until the light flashes rapidly—this clears all pairings. Then start the process over.
Heads Up
The soundbar can only remember one paired device at a time. If you pair a new phone, it forgets the old one. You'll have to re-pair if you want to switch back.