Those manuals are not 100% up to date in some cases like this one.
Tags must be added before hand, and then when you edit the task Relations, then you can relate it to the tags.
Custom reports do filter custom properties, but truth be told I do not remember by heart right now whether this is available for the Community Edition or if it is only for the Professional Edition. In any case, I suggest using the latter for business oriented organizations.
Best regards,
Francisco