R Shinydashboard: How to reference box in conditionalPanel() condition argument

I have a Shiny app where I would like to set a conditionalPanel() depending on whether the user selects a tabPanel from a tabBox. My question is how can I reference the panel the user selected. In my example the idea is that if the user selects "Panel1" there appears a conditional panel. As you will see I tried to set the condition in the conditionalPanel() call as condition = "input.tabs.tab1.box == 'p1'".

library(shiny)
library(shinydashboard)

ui <- dashboardPage(
  skin = "green",
  dashboardHeader(title = "Referencing Box"),
  dashboardSidebar(
    width = 250,
   sidebarMenu(
     id = "tabs",
     menuItem("First Tab", tabName =  "tab1",
              menuSubItem("Box", tabName = "box"))
   )
  ),
  dashboardBody(
    tabItem(
      tabName = "box",
      tabBox(
        id = "tb",
        title = "Two Panels",
        tabPanel(
          id = "p1",
          title = "Panel1"
        ),
        tabPanel(
          id = "p2",
          title = "Panel2"
        )
      ),
      conditionalPanel(
        condition = "input.tabs.tab1.box == 'p1'",
        box(
          id = "p3",
          title = "Conditional Panel"
        )
      )
    )
  )
)

server <- function(input, output) { }

shinyApp(ui, server)

I've seen some examples around where people referenced the upmost layer, so a menuItem with conditions like condition = "input.tabs == 'anova'", but in this case the referenced item is within a tabItem and further within a tabBox call.