Cant make glare work while compositing I have a simple monkey model, that is completely black but has white, emissive eyes, like that:

Then, in compositor, I have this setup:

Clearly, you can see the preview is not showing any glare effect. I think, that I'm missing something there, but for the sake of me I can't figure out what.
Try lowering the Threshold value.
The Glare node works based on the luminance (or brighness) of the pixels in the image, the Threshold value means that pixels that are brighter than that number will be affected by the glow, while values lower than the set number will not.

By lowering it, you are changing the node's level of tolerance. If it's set to a low value, it means that even a small amount of light will make the glare show up.
You also may be rendering in EEVEE. Switch to Cycles.