This really depends on if you've got any existing comics.
If you have another comic you've made before, use that as your main line of "attack" for marketing.
If you haven't, it's going to be very difficult. After all, people are giving money to a creator they don't know will deliver. They don't know what kind of quality they will get.
Make a thread here in the forums if you need, but don't overdo it or you could come across as pushy. Use any work you do have here to promote - put it on your wall, make an update for an existing work you have talking about it... You could also send something to your subs so long as you're not too pushy.
Webtoon isn't suitable for any kind of promotion at all. There's no equivalent forum function and barely any social links between readers and creators.