How to write an email to Horikoshi?

Hello, I would like to write an email to Kohei Horikoshi, the author of My Hero Academia, but I still don't understand how to do it. Does anyone here know how to write him an email?
And another question: is it possible to contact Horikoshi directly from his Instagram account?
Thanks everyone for the replies.

The answer to your second question is pretty easy to find out yourself. Have you tried contacting him via Instagram? It is usually pretty easy to tell whether someone wants to be contacted there or not. Note that most famous people do not.

Next best option might be to go through his publisher.

ファンレター、ご意見、ご感想を著者に送りたい
回答
本の後ろのページにある編集部の住所宛てにお送りください。
〈例〉
〒101-8050
東京都千代田区一ツ橋2-5-10
〇〇編集部 △△先生宛

you can find the (publisher's) address on the last page of the book.

I notice that Mr. Horikoshi posts frequently both on Instagram and Twitter, and there are lots of fans commenting in various languages on his posts. I believe it's the easiest and best way to send messages to him. He can use automatic translators to get the gist of the meaning.

And just because he doesn't "follow" you or "like" your comments, that doesn't mean he doesn't read them. Quite often, famous people secretly check fan comments, or at least their manager would check them on their behalf and pass on nice messages.