How do I texture paint with glow? I found someone doing this and I wondered how you do it in blender 2.8. Preferably in cycles as I need to texture bake the final result.

I'm willing to add more details if needed.
It has nothing to do with the painting per se.

First, you need to paint the emission map of your shader :


Make sure to use black as blank color when creating the texture, so it doesn't emit light when nothing is painted.
I don't use the Principled BSDF's emission here because there's no strength input.

The glow itself comes from his viewport which is in rendered mode:



And using Eevee which has a bloom effect that can be seen in the viewport:



You can do the same with Cycles, except it doesn't have any SFX visible in the viewport, you would need to do it through compositing using a glare node in fog glow mode: