我不确定post，但这里有一个RSelenium的方式来下载文件：

# Load library ---------------------------------------------------------------
library(RSelenium)

# The URL changes depending on what parameters are chosen
# So we don't have to use RSelenium to set parameters!
# It can be done with the URL

# Choose values------------------------------------------------------------

transaction_type <- "Futures"
symbol <- "CRA"
start_date <- "2023-03-16"
end_date <- "2023-03-16"

url <- paste0("https://www.m-x.ca/en/trading/data/historical?symbol=",
              symbol,
              "&from=",
              start_date,
              "&to=",
              end_date)

# I think this bit just specifies where on the page we start, so probably not
# needed:     #CRAH23-20230316

# start RSelenium ------------------------------------------------------------

# set up RSelenium ------------------------------------------------------
rD <- rsDriver(browser="firefox", port=4547L, chromever = NULL)
remDr <- rD[["client"]]

# Navigate to webpage -----------------------------------------------------
remDr$navigate(url)

# Click on the download button -----------------------------------------------------
remDr$findElements("id", "btnDnld")[[1]]$clickElement()