It's informal but totally fine in conversation. In this sentence, it sounds fine because “写(这个）字” is a phrase that makes sense. But if i say 怎么走这个地方, that will sound weired. 这个地方怎么走 would be the right to say
It would be very helpful if I could click on the characters after I have answered wrong and hear what they are. I see the characters for the correct answer, but often I don't know what they are. And once I have answered wrong, clicking on the characters tells me nothing.