Is there an equivalent of Retrofit.Builder class in Ktor? See https://square.github.io/retrofit/2.x/retrofit/retrofit2/Retrofit.Builder.html

Sorry, I think you need to specifically tell the target you want to achieve. If you just want to add some pre-config before calling apis at Ktor construction step: check examples from Plugins(formerly known as Features) https://ktor.io/docs/default-request.html Especially, the Defatult Request Plugin.