In R shiny, how do I add functionality to the action button "Add"?

Question

I am working on the R shiny app. I have created Tab1 and Tab2 in Shiny, and what I am attempting to do is make the datatable visible in "Tab2" once I fill in the below forms and hit the "Add" button.

Summary

When the user enters information on "SchoolId, UserId, class, result" in Tab1 and clicks the "Add" button, the Datatable in "Tab2" should appear.

Also, once the user presses the "Add" button, the fields "SchoolId," "UserId," and so on should be empty, ready for the user to fill in the data of the second row in "Tab2."

Issue:

I am sure that I am missing something here to build a server-level connection for the "Add" button, because I can't fill in the details for the second row in Tab2 because since "Add" button is not working.

Could someone help to fix this issue?

library(shiny)
library(shinyBS)
library(stringr)
library(Hmisc)
library(rhandsontable)
library(shinydashboard)
library(DT)
library(V8)

ui <- fluidPage(
  fluidRow(tabsetPanel(id='tabs'
                           , tabPanel("Tab1", uiOutput("tabP1"))
                           , tabPanel("Tab2", uiOutput("tabP2"))
    )
   
  )
)

server <- function(input, output, session) {
 output$tabP1 <- renderUI({
    tabPanel("Tap1"
             , div(id = "form"
                   , textInput("schoolId *", label="SchoolId" )
                   , selectInput("userId", label="UserId"
                                 , choices = list("UserA" = "UserA", "UserB" = "UserB",
                                                  
                                                  "UserC" = "UserC")
                                                  
                                                  , selected = "UserA")
                   
                   
                   , textInput("class", value = NA, label = "class")
                   
                   , selectInput("result", label="result"
                                 , choices = list("PASS" = "PASS", "FAIL" ="FAIL" ), selected = "")
                  
             )
             
             , actionButton("add", "Add")
    )
  })
  
 
  # -------------------- Tab2  ----------------------------------
  
  observeEvent(input$add,{
    req(input$schoolId,input$userId,input$class,input$result)
    tmp<-data.frame(SCHOOLID=input$schoolId, USERID=input$userId
                    , CLASS= input$class
                    , RESULT=input$result
                    
    )
    
    df <- rbind(df,tmp)
  })
  output$DT2 <- DT::renderDataTable({
    df
  })
  
  output$tabP2 <- renderUI({
    tabPanel("View"
             , DT::dataTableOutput("DT2")
             , hr()
             , hidden(downloadButton('downloadData', 'Download'))
             
    )
  })
  
}

shinyApp(ui, server)