It sounds like you're using some kind of TODO addon? The feature you're describing isn't something built into HipChat itself, but provided by a 3rd party integration. I would suggest trying just the "/todo" slash command alone, which might give you a list of commands you can use (and maybe one of them allows you to complete the todo).
If that doesn't work, I would recommend contacting the addon creator directly. You can find addons by searching on https://marketplace.atlassian.com/home/hipchat. It's possible that the option you're looking for exists, but is hidden somehow. Otherwise, you can always suggest that it get added in a future update.
Hopefully that helps answer your question!
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