So, I was able to reach the API in curl using:
curl -X GET -k --header "x-apikey: accesskey=XXXXX; secretkey=XXXXX" https://scnessus.redacted.org/rest/currentUser
But the translation to R just gives me a "Resource is not supported." error message. I don't understand why it would work through command line but not through RStudio?
> require(httr)
> headers = c('x-apikey' = 'accesskey=XXXXXXX; secretkey=XXXXXX')
> tenable <- httr::POST(url='https://scnessus.redacted.org/rest/currentUser',
+ httr::add_headers(.headers=headers), config = httr::config(ssl_verifypeer = FALSE))