Redmine › Hooks Manager

I tried to follow the tutorial for adding a new placeholder/hook given here. I decided to just follow the path of editing the Hooks Manager plugin files rather than creating my own plugin (yes, I understand the consequences and I have the files backed up).
I successfully added the hook to where I can see it in the hooks manager plugin configuration on the web client as administrator. But trying to add any content to the hook results in a “Hook is invalid” error (see attached). The hook I’m trying to add is one that goes beneath the right column hook for the project overview view. I can’t figure out what (if anything) I did wrong.

What results in the “Hook is invalid” error? If any additional info is needed, I will provide.

Thanks.