Well, to say from experience, I was chosen for the "Specials/Unique" Sections for the app (which shows up as the intro to the Canvas section on desktop). As to why I was picked, I still don't know (like, Webtoons probably knows, but I personally don't have that answer).
Anyhoo, my promotion lasted a week, which was a nice promotion I'll say. Bumped me up to 2k subs and gave me a bit more activity. Though, I will say that before I was picked, I was also posting on Webtoons for a few months with another comic.
The number of episodes doesn't seem relevant -- they picked me when I was around 5-6 episodes, but someone else along with me had only 3, and another had more than 10. For my experience, they just use the icon I was already using for my comic.
Though again -- I don't think it's entirely random. I've seen a lot of others picked that I know from here, and they (just like me) have been posting for a while, might have a few works under their belts, and are consistent.