How to dynamically populate dropdown box choices in shiny dashboard

I am developing one app in shiny dashboard in that I want to dynamically populate dropdown box once csv is uploaded. Dropdown will contain top 10 cities by user registrations which I get from following code.

final_data %>%  group_by(registrant_city) %>%  summarise(Total = n())    %>%  arrange(desc(Total))      %>%  top_n(n = 10)  

These cities should go into dropdown box.

tabItem("email",          fluidRow(            box(              width = 4, status = "info",solidHeader = TRUE,              title = "Send Emails",              selectInput("email_select", "Select Email Content",                          choices = c("Price" = "price","Services" = "service"                                      )),              selectInput("cities", "Select City",                           choices = ??                          ))                         ))

Please help..