I'm using a Java lib which internally uses Apache HttpClient 4.3 for sending https requests. The 3rd party server requires the 'Content-Type' header which is unfortunately not set by the lib.
As changing the lib is not an option, I'd like to use javaagents to add the header.
I found this useful tutorial which made me believe it's possible to achieve this: https://httptoolkit.tech/blog/how-to-intercept-debug-java-http/ But I couldn't figure out which interface of HttpClient 4.3 to manipulate to set the header. Does anybody know how it could work?