It sounds like you want to navigate to another screen to show the popup. If navigating is not a necessity, I would suggest you add the popup, which I assume is a text box or label that becomes visible when xxxx is in the control, to the same screen as the drop down.
To set the pop-up on the same screen, I would set the visible for the pop-up to If(Self.Selected.Value = "Law Enforcement" , true, false)
For it to navigate to another screen, like @WarrenBelz mentioned, on the data card On Change property to:
If(Self.Selected.Category = "Law Enforcement",