There's a nice tutorial here: http://content.photojojo.com/tutorials/create-your-own-panorama-planets/
Or another one here: http://www.tech-recipes.com/rx/36616/photoshop-cs6-create-the-little-planet-effect-from-a-panorama-picture/
The key is to resize your document (the image itself, not the canvas) into a square image first. The panorama image will look horribly distorted beforehand, but it will come out OK after the filter.
Image used as an example is Panorama of Helsinki, by bearinthenorth, from pixabay
I signed up for the public beta, and was very impressed with the features and functionality. I do have issue that has nothing to do with 3rd party plugins, but with Affinity itself. Will post it appropriate forum.