Example for making a modal with box elements and a selectInput

app.R

library(shiny)
library(shinydashboard)

# Define UI for application that draws a histogram
ui <- dashboardPage(title = "Simple dashboard",
                    header = dashboardHeader(),
                    sidebar = dashboardSidebar(),
                    dashboardBody(actionButton("open_modal", "Open modal"),
                                  textOutput("chosen_fruit"))
)

# Define server logic required to draw a histogram
server <- function(input, output) {
    
    output$chosen_fruit <- renderText({
        
        req(input$some_input)
        
        paste0("You have chosen: ", input$some_input)
        
    })
    
    observeEvent(input$open_modal,
                 showModal(
                     modalDialog(
                         tagList(box(title = "First box", status = "primary", width = 12,
                                     selectInput("some_input",
                                                 "Select here",
                                                 choices = c("Orange", "Apple", "Paer"))),
                                 box(title = "Be sure!", status = "warning", solidHeader = T, width = 12,
                                     "Choosing fruits is not an easy task. Be sure of your selection!"
                                     )),
                         title = "A simple modal", size = "l", easyClose = F)
                     )
                 )
}

# Run the application 
shinyApp(ui = ui, server = server)