This example use HTML templates to render Shiny UI. The template is modified from https://github.com/amrrs/rshiny_html_template. As you can see in the HTML file, you can directly use R function in the placeholder.

The search bar is written in HTML, but you can access it in Shiny by its “name” attribute, for example a input tag like

<input type="text" class="form-control" placeholder="Search"  name="search">

Its value can be extracted in server function simply by

server <- function(input, output, session) {
  ...
  output$searchInput <- renderText(input$search)
}

If you try input some text in the search box, it will display here.