Thank you so much! Never knew that was even possible!
To anyone also looking for this in the future, here's what I did:
1. 1/4 to 1/8 into Sustain Port.
2. Aux cable (3.5mm) to 1/8 into PC headphone port. (You need an input device for the aux, a phone or anything with a aux port will work.)
That should be all! For the PX350, there's an issue though. The sustain will not stay on when restarting or hitting "Song Recorder".
The fix for that is just to plug out either of the inputs and plug it back in, and it will reactivate instantly. It's a minor annoyance, but works.