Going to a project workboard such as Projects and using the pencil icon in a column to create a new task, but right click and open in a new tab, does not prefill the tags input with the project tag associated with that workboard.

If you go to a Workboard, and you open its context menu, and you hover your mouse on the Create Task action, you see this URL or a similar one:

http://example.com/maniphest/task/edit/form/default/

The proposed change is to have this URL or a similar one instead:

http://example.com/maniphest/task/edit/form/default/?tags=test

Note that the destination form already handles the tags= parameter so this would just be a minimal frontend change, just to help people who like to open links in a new tab.