How do I create feedback?

Turn you monitors up and put your pickups closer to the monitors.

However, the type of feedback you're trying to get is the sound of a large real amp/cabinet on stage. The feedback you'll get through a plugin and small full-range monitors will probably start off as an uncontrollable, very sudden and loud, very high-pitched whine. You might need to do a lot of experimentation with different patches and guitar placement to get the natural sound you are looking for. I'm sure you can download some samples of feedback, which might suffice if it's just for one song.

If you want feedback regularly, and especially live and at lower volumes, I've got a guitar pedal called a Digitech Freqout. They've stopped making them now, but you might be able to pick one up second hand. I love it because it can mimic feedback at low volume levels and is very controllable. Because of people's psychoacoustic perception of real feedback, it can give a sense of power and chaos where there is none. It also does a bubbly, atmospheric Theremin sound too, which sounds quite unique. One word of warning though - it only has the capacity to make any sound for ten seconds. 90% of the time, this is enough in the context of a song, but there are times when I wish it would sustain for longer (like an e-bow). It must be due some limitation on its motherboard. Digitech don't exist any more, so there's no chance of firmware updates for any of their pedals (which is a small reason to be careful buying any Digitech stuff left on the shelves).

 

Late reply, but hey...
I have a Freqout pedal, it's great - if I put it in my pedal chain before the overdrive, its output gets distorted, so it really sounds like the amp is going out of control.
There's an equivalent in software - Blue Cat Audio has a VST called AcouFiend: https://www.bluecataudio.com/Products/Product_AcouFiend/
It's not cheap, for one effect, but if you REALLY want that effect...
$0.02