There are many libraries for xAPI but my experience is with TinCanPHP.
I use Learning Locker as the LRS and through the VaRest and HTTP requests plugin you can set variables from within UE4 and then POST the xAPI statement.
I chose PHP library because I have several existing mySQL dbs with user info stored. A simple GET request grabs those values and sets them as UE4 variables. Then, when you POST the xAPI statement you can use the UE4 variables as part of the POST data in the PHP file.