API 1.1 requesting twitter bearer token using r - r

Question

I have searched this forum and tried several things that seemed relevant, but with no success. If anyone can spot what I'm missing I would be very grateful.
I am trying to get a bearer token using application only authorization as explained at https://dev.twitter.com/docs/auth/application-only-auth so that I can GET follower s/ids.
I have constructed a request in r using rstudio with my consumer key & secret in Base64 encoded form.
library(httr)
POST(url="https://api.twitter.com/oauth2/token", config=add_headers(
c('Host="api.twitter.com"',
'User-Agent="NameOfMyApp"',
'Authorization="Basic MyKeyandSecretBase64Encoded"',
'Content-Type="application/x-www-form-urlencoded;charset=UTF-8"',
'Content-Length="29"',
'Accept-Encoding="gzip"')), body="grant_type=client_credentials")
In response I receive:
Response [https://api.twitter.com/oauth2/token]
Status: 403
Content-type: application/json; charset=utf-8
{"errors":[{"label":"authenticity_token_error","code":99,"message":"Unable to verify your credentials"}]}
I tried resetting my credentials but it made no difference.

Answer 1

I'm a few weeks late, but for anyone like me who stumbles across this page, here is some code that works for me, returning a bearer token:
POST(url="https://api.twitter.com/oauth2/token",
config=add_headers(c("Host: api.twitter.com",
"User-Agent: [app name]",
"Authorization: Basic [base64encoded]",
"Content-Type: application/x-www-form-urlencoded;charset=UTF-8",
"Content-Length: 29",
"Accept-Encoding: gzip")),
body="grant_type=client_credentials")
Once you have a bearer token, you put it in the header of a GET like so:
GET("https://api.twitter.com/1.1/followers/ids.json?cursor=-1&screen_name=justinbieber&count=5000",
config=add_headers(c("Host: api.twitter.com",
"User-Agent: [app name]",
"Authorization: Bearer [bearer token]",
"Accept-Encoding: gzip")))

Answer 2

A late response, but the existing answer wasn't working for me. So here's a solution with a modification of the GET request.
add_headers() uses a named vector. This requires the hyphenated header names to be bracketed with backticks (``). So your POST() call should be:
response <- POST(url = "https://api.twitter.com/oauth2/token",
config = add_headers(.headers = c(Host = "api.twitter.com",
`User-Agent` = "NameOfMyApp",
Authorization = "Basic [base64encoded]",
`Content-Type` = "application/x-www-form-urlencoded;charset=UTF-8",
`Content-Length` = "29",
`Accept-Encoding` = "gzip")),
body = "grant_type=client_credentials")
Within a successful response the application access token can be accessed with:
bearer_token <- jsonlite::fromJSON(rawToChar(response$content))$access_token
You can then verify this with a GET request, such as:
GET("https://api.twitter.com/1.1/followers/ids.json?cursor=-1&screen_name=justinbieber&count=100",
config = add_headers(.headers = c(Host = "api.twitter.com",
`User-Agent` = "NameOfMyApp",
Authorization = paste("Bearer", bearer_token),
`Accept-Encoding` = "gzip")))