To enable "Related To" for Daily Log items, so that you can link Daily Log items to other items.
Navigate to your project's Daily Log tool.
Click Configure daily log settings... in the right pane.
Check the "Show Related To On Daily Log" checkbox.
Return to the Daily Log tool by clicking Back.
Click the orange Enable Related To button in the right pane.
There will be a new "Relate To" field at the top of the page.