With any sort of freelancing, they'd be making a new contract with each artist, I assume. So, just make sure you are aware of standard rates that Tapas or Webtoon have before you apply by doing some cursory google checks. I know about standard rates for the Big 2...not sure about online webcomic rates. (also, for each job, you'll have a different rate, depending on difficulty and how much work needs to be done, and whether they need it rushed or not.)
sorry the answer is we don't have an answer, but you really can't know until you apply and they tell you what the job is.