library(httr)
library(base64enc)
library(jsonlite)

url<-"https://api.ebay.com/identity/v1/oauth2/token"
clientID<-"****-testapp-******-*****"
clientSecret<-"PRD-8********-***-***-****-****"
base64encode(paste(clientID,clientSecret,sep = ":"))


request_body <- data.frame(
  code="v%5E1.1%23i%5E1%23I%5E3%23r%5E1%23p%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
  grant_type="authorization_code",
  redirect_uri="*****-*****-testap-******"
)


##Approach 1##
result <- POST(url=url,
               body = request_body,
               add_headers(.headers = c("Content-Type"="application/x-www-form-urlencoded",
                                        "Authorization"="Basic ******************...*****YQ=="
               )))
Output <- content(result)

##Approach 2##
request_body_json <- toJSON(list(documents = request_body), auto_unbox = TRUE)

result <- POST(url=url,
               body = request_body_json,
               add_headers(.headers = c("Content-Type"="application/x-www-form-urlencoded",
                                        "Authorization"="Basic ******************...*****YQ=="
               )))

Output <- content(result)
result
Output <- content(result)
Output

> result <- POST(url=url,
+                body = request_body,
+                add_headers(.headers = c("Content-Type"="application/x-www-form-urlencoded",
+                                         "Authorization"="Basic RGVubmlzRXUtdGVzdGFwcC1QUkQtMzhlMzVjNTM1LWFjNzdjODE5OlBSRC04ZTM1YzUzNTFiNzktN2UwMS00YzMxLWE0YTQtM2U5YQ=="
+                                         )))
Fehler in curl::handle_setform(handle, .list = req$fields) :
  Insupported value type for form field 'code'.

> Output <- content(result)
> Output
$error
[1] "unsupported_grant_type"

$error_description
[1] "grant type in request is not supported by the authorization server"