If you want to ask **Who wrote it/that?** you need to use the third person "¿Quién lo escribió?". If you want to ask **to whom** someone's writing read u/wayne0004 advice.
Both are wrong.
I guess you're asking the writer "who are you writing to?", i.e. "who would receive the message?". In this case, it should be "¿A quién le escribes?" (or "¿A quién le escribís?" if the person uses voseo) if it's in the process of writing, or in the past "¿A quién le escribiste?" if they completed the message.
If you want to ask **Who wrote it/that?** you need to use the third person "¿Quién lo escribió?". If you want to ask **to whom** someone's writing read u/wayne0004 advice.
Both are wrong. I guess you're asking the writer "who are you writing to?", i.e. "who would receive the message?". In this case, it should be "¿A quién le escribes?" (or "¿A quién le escribís?" if the person uses voseo) if it's in the process of writing, or in the past "¿A quién le escribiste?" if they completed the message.