Does this work? I.e. change the URL and get the changes

enableBookmarking("url")

sources <- c("ECDC", "JHU", "JHU_US")

get_countries <- function(source = "ECDC") {
  switch(source,
         "ECDC" = c("Spain", "Switzerland"),
         "JHU" = c("USA", "Australia"),
         "JHU_US" = c("Alabama", "Florida"))
}

shinyApp(
  
  ui = function(req) {
    fluidPage(
      textInput("txt", "Text"),
      checkboxInput("chk", "Checkbox"),
      selectInput("source", "Choose a data source:",
                  sources),
      uiOutput("country_input")
    )
  },
  server = function(input, output, session) {
    
    output$country_input <- renderUI({
      selectInput("country_output", "Choose a country or region:",
                  # selected = "Switzerland",
                  get_countries(input$source))
    })
    
    observe({
      # Trigger this observer every time an input changes
      reactiveValuesToList(input)
      session$doBookmark()
    })
    
    onBookmarked(function(url) {
      updateQueryString(url)
    })
    
  }
)

I found the information presented here to be remarkably useful, providing practical tips that I can easily apply in my life. @ Retro bowl