Have task 7 day lag before another task

[looknow12]

I'm using Primavera P3. I have one central task with predecessors and successors and which will shift throughout the project.

I need to create a task which will always be 7 days before the start of this other task.

I have tried a SS and SF with a -7D lag. But it does not seem to work (the date of this new task does not seem to automatically calc 7 days prior)

 

[JohnTheKey]

Is the "task which will always be 7 days before the start of this other task" a milestone or an activity?
Does it require resources?
Does it have a variable duration?
JTK