to fill in (field, entry, etc.); to fill out (form)
to post a message (e.g. on a bulletin-board)
to store
《computing》to write (data); to flash
名前を書き込んでください。
Please fill in your name.
日記にその日の出来事を書き込んだ。
I wrote the day's events in my diary.
書き込む欄が足りない。
There's not enough space to write.