library(httr)
xml <- "some XML content"
response = POST(url ="https://xyzhost.com",
                content_type_xml(), accept_xml(),
                config = config(sslcert = "/path/to/cert.pem",
                                sslkey = "/path/to/privatekey.pem",),
                body = as.character(xml), verbose())

-> POST /query HTTP/1.1
-> User-Agent: libcurl/7.29.0 r-curl/3.1 httr/1.3.1
-> Host: xyzhost.com
-> Accept-Encoding: gzip, deflate
-> Cookie: JSESSIONID=long_string; CTPSESSION=long_str //<---missing on Windows
-> Content-Type: application/xml
-> Accept: application/xml
-> Content-Length: 482