I'm using HOCON to configure log messages and I'm looking for a way to substitute placeholder values dynamically.

I know that ${?PLACEHOLDER} will read an environment variable and returns an empty string when the PLACEHOLDER environment variable doesn't exist.

Example

This is an example of what I had in mind:

(I'm using config4k to load HOCON )

data class LogMessage(val message: String, val code: String)

fun getMessage(key: String, placeholderValues: Array<String> = arrayOf()): LogMessage {
    val config = ConfigFactory.parseString("MY_LOG_MESSAGE {code = ABC-123456, message = My log message with dynamic value %0% and another dynamic value %1% }")

    val messageObject = config.extract<LogMessage>(key)
    var message = messageObject.message

    placeholderValues.forEachIndexed { placeholderIndex, value ->
        message = message.replace("%$placeholderIndex%", value)
    }

    return messageObject.copy(message = message)
}

fun main(args: Array<String>) {    
    println(getMessage("MY_LOG_MESSAGE", arrayOf("value 1", "value 2")))

    // prints: LogMessage(message=My log message with dynamic value value 1 and another dynamic value value 2, code=ABC-123456)
}

Even though this works, it doesn't look like the best solution and I assume there already is a solution for this.

Could someone tell me if there is a built-in solution?

First things first.
HOCON is just a glorified JSON format. config4k is just a wrapper.
All your work is being done by Typesafe Config, as you've probably noticed.
And judging by their documentation and code, they support placeholders only from withing the file, or from the environment:

This library limits itself to config files. If you want to load config from a database or something, you would need to write some custom code.

But for what you're doing, simple String.format() should be enough:

fun interpolate(message: String, vararg args: Any) = String.format(message, *args)

println(interpolate("My message was %s %s %s %s", "a", 1, 3.32, true)) 
// My message was a 1 3.32 true

Notice that you can use * to destructure your array.