1.
If you write in to an organization, you send them a letter.
What’s the point in writing in when you only print half the letter anyway?...
So there’s another thing that you might like to write in to this programme about.
PHRASAL VERB : V P , V P to n
2.
In the United States, if someone who is voting in an election writes in a person whose name is not on the list of candidates, they write that person’s name on the voting paper and vote for him or her.
I think I’ll write in Pat Wilson...
I’m going to write him in on my ballot next year.
PHRASAL VERB : V P n (not pron) , V n P
3.
see also write-in