library(shiny)
library(shinydashboard)
library(shinyjs)

tabItem_welcome <-
  tabItem(tabName = "welcome",
          textInput("welcome_text", label=NULL, value = "")
  )
tabItem_a <-
  tabItem(tabName = "a", id = "a",
          h2("A")
  )

dashboardPage(
  dashboardHeader(title = "test"),
  dashboardSidebar(
    sidebarMenu(id = "tab",
      menuItem("Welcome", tabName = "welcome"),
      menuItem("A", tabName = "a")
    )
  ),
  dashboardBody(
    useShinyjs(),
    tabItems(
      tabItem_welcome,
      tabItem_a
    )
  )
)

shinyServer(function(input, output) {

  hide(id = "a")

  observe({
    welcome_text <- input$welcome_text
    if(welcome_text=="show a"){
      show(id = "a")
    }
  })
})