You can't make questions community wiki anymore, this ability is restricted to diamond moderators now. The community wiki feature on SE is not exactly intuitive, often misunderstood and was mostly used in ways it wasn't actually intended to work.
There is no good reason to make a question community wiki at all on mosts sites now. Questions are generally not collaboratively edited, so the wiki part isn't useful. And the reputation denial only discourages users from answering as they won't get any reputation for it.
Community wiki also isn't an excuse to post otherwise off-topic questions, simply because you don't earn reputation for them. Either questions are on-topic, and everyone should earn reputation for them, or they're not and shouldn't be posted at all.