What is the recommended way to add third party library?
this lib is a .jar file for send message with custom feature
.jar is for Android, Android is a native platform, Ionic uses Cordova for the native platforms stuff.
Cordova has plugins that can do stuff with files, like copying them to specific paths etc.
So I think your best chance is to create a custom Cordova plugin that includes the .jar and copies it to where it belongs. The JS code then can also make sure you can actually use the functionality of it.
I would look at plugins that do something like that and adapt it. I know some tracking SDK Cordova plugins include .jar files that are copied somewhere.