I am not sure if you are asking the same question that I am interested in. I have setup a generic notification requesting a status for a given task. I want to send an email to the ($users) assigned to the task, but the mail notification template will not allow me to use Tags in this field. At any given time, I want to use the "send once:now" notification within a task I would like an update for. Any notifications I have created thus far, have been sent to a specific user.
If you are trying to embed a link within the notification itself, you could use the statement below:
Link to Task Details.
I pulled this from the "Customize" "Customized Notifications" "Email body to task user"