What's the preferred library for sending HTTP requests? I don't need anything fancy, just basic cookie/session functionality and minimal setup

ktor-client if you want to use something kotlin-native. Retrofit is pretty good otherwise. There are a bunch of other Java options out there if one of those doesn’t suit your needs.

Which Java library would you recommend? It doesn't have to be Kotlin specific if the Java libraries have better docs

Retrofit is a wrapper over OkHttp, which would be my recommendation in that case

If you want the simplest thing possible, try something like the apache fluent http client. Which is a little bit weak overall but easy to get started with IMO. (OkHttp might also be good for that, I haven’t used it directly before).

It seems to me that Retrofit is a lot more "enterprise" and requires more setup. I just need to send a login request, then use the returned session cookie to send another http request. I'll take a look at OkHttp

With Retrofit you just have to set up an interface and a class that creates the service using a builder https://square.github.io/retrofit/