To Write In, To Fill Out, To Jot Down
transitive verb, godan verb
You know that 書く is "to write" and the kanji 込 is into, so this means to write in, to fill out, or to jot down. 書き込む focuses more on the physical action of writing than 書く on its own. This is because 込む stresses that you’re putting words INTO some space, either on paper or on computer.
かきこむ
The reading comes from the word 書く and the kanji 込 combined into one.
急いで申込書に書き込んだ方がいいよ。あと十分で事務所は閉まっちゃうんだから。
You better hurry up and fill out an application. The office will close in ten minutes.