if it is a random webnovel, contact the author with a comment on the novel if it is a novel in some publishing website (qidian, munpia, whatever the main japonese one is), then you would have to contact both their publishing and legal department, either prepare a massive check for them , prepare to be served a contract and give way the rights to your translation or simply be given a cease and desist my advice, start translating without lincence, if you get big enough to be noticed the worst that can happen is reieving a DMCA, that does not do much if you comply or you can then make a deal with the company
Contact the author on the raws website. If the author is in contract already to either Qidian, Munpia or Jnovel then expect that either you cannot translate it or you'll need to contact the publisher(which means you will need to pay a ton or get denied also).
If chinese, likely you need to refer to the websites/platform they are hosted (like Qidian). If japanese, you'll either need to contact directly the author (if not-published, likely you'll contact him on syosetu via message) or contact the publishing houses (like KADOKAWA, KODANSHA, etc) but then in this case, it's just impossible cuz big corporations. if korean, you'll have to deal with "Managements" (Munpia management dept., Raon E&M, etc) or publishing houses (Papyrus, Seed Novel, etc.) most of the time but in any case, nearly impossible as well to get anything unless you're a corporation buying publishing rights. (tho with independent authors, you have a tiny bit more luck and it seems to work very well among the BL/yuri novel community)
If it's not owned by a corporate entity, contact the author directly. If it is owned by a corporate entity, prepare a lot of money, then send a mail to their general affairs or business section. Helpdesk will likely not be able to help you as negotiating deals is not part of their job.
Contact them on twitter or in the comments of the novel, usually works. Of course, that's only for unpublished webnovels.
For those that do get permission to make fan translations, make sure you keep a screen shot of the message of the writer and/or giving their approval and perhaps any file you might have of that permission, such as a PDF or Word version. There is the off-chance that the author might either deny having giving you permission or forget that they gave you permission. Also, be sure to comply with any possible conditions you were given. This could be that you might not be able to use the work commercially. Also, understand that should the work get licensed in your home country, it might be best to stop translating the work to avoid any possible legal confusion or issues.
If you aren't looking to sell your translations as an ebook, don't bother with asking for permission. Most Chinese webnovel authors are not exactly professionals. They're bored high school students and office workers who only have a bit more talent at writing than the average joe. Nobody's going to come after you if you don't have permission, either. Chinese publishing houses have bigger things to worry about than some random person posting their translations on their blog site for free, halfway across the world.