Add an icon to selectInput dropdown menu

I want to add an icon even like a small triangle just so the user knows that it is a dropdown menu. I see it is there if it the user is just selecting one pick, in my app they can select multiple items in the list so it doesn't show the arrow.
UI:

selectInput("traffickingSubType", "Choose a trafficking sub type: ",
                      choices = sort(unique(newngo$Trafficking.Sub.Type)), selected = NULL,
                      multiple = TRUE, selectize = TRUE, width = NULL, size = NULL)

Server:

output$Sub <- renderPlotly({
    ggplot(filteredData(), aes(x=Trafficking.Sub.Type)) + geom_bar()
  })

If there is a way something added and keep the multiple choice options that would be great!