Thanks for the welcome and I appreciate your thoughtful response 
Yes I am aware the issue here lies with the browser—indeed the web app treats all custom links the same, but when I use custom links in Chrome, there is no added about:blank page (or it appears and then is automatically deleted by Chrome within a millisecond). I tested this using multiple web apps on each browser and my findings were consistent.
Incidentally, my task manager of choice is ClickUp, and your workaround for entering hook links into custom fields is brilliant 
Hook is proving to be such a task-management game-changer, I just wish I could get around these blank Firefox tabs!
Maybe I’ll just have to live with them ? (I’m not going back to Chrome…)