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!
Earlier this year we made the difficult decision to discontinue our team messaging tools, Stride and Hipchat. The end of life date for Stride and Hipchat Cloud is quickly approaching - February 15, 2...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs