How do you pass parameters to a shiny app via URL

You'd have to update the input yourself when the app initializes based on the URL. You would use the session$clientData$url_search variable to get the query parameters. Here's an example, you can easily expand this into your needs

library(shiny)

shinyApp(
  ui = fluidPage(
    textInput("text", "Text", "")
  ),
  server = function(input, output, session) {
    observe({
      query <- parseQueryString(session$clientData$url_search)
      if (!is.null(query[['text']])) {
        updateTextInput(session, "text", value = query[['text']])
      }
    })
  }
)

Building off of daattali, this takes any number of inputs and does the assigning of values for you for a few different types of inputs:

ui.R:

library(shiny)

shinyUI(fluidPage(
textInput("symbol", "Symbol Entry", ""),

dateInput("date_start", h4("Start Date"), value = "2005-01-01" ,startview = "year"),

selectInput("period_select", label = h4("Frequency of Updates"),
            c("Monthly" = 1,
              "Quarterly" = 2,
              "Weekly" = 3,
              "Daily" = 4)),

sliderInput("smaLen", label = "SMA Len",min = 1, max = 200, value = 115),br(),

checkboxInput("usema", "Use MA", FALSE)

))

server.R:

shinyServer(function(input, output,session) {
observe({
 query <- parseQueryString(session$clientData$url_search)

 for (i in 1:(length(reactiveValuesToList(input)))) {
  nameval = names(reactiveValuesToList(input)[i])
  valuetoupdate = query[[nameval]]

  if (!is.null(query[[nameval]])) {
    if (is.na(as.numeric(valuetoupdate))) {
      updateTextInput(session, nameval, value = valuetoupdate)
    }
    else {
      updateTextInput(session, nameval, value = as.numeric(valuetoupdate))
    }
  }

 }

 })
})

Example URL to test: 127.0.0.1:5767/?symbol=BBB,AAA,CCC,DDD&date_start=2005-01-02&period_select=2&smaLen=153&usema=1