The DSL looks like this:
val onboarding = guild.editOnboarding {
    enabled = true
    mode = OnboardingMode.Advanced
    defaultChannelId(channelId1)
    defaultChannelId(channelId2)
    defaultChannelId(channelId3)
    defaultChannelId(channelId4)
    defaultChannelId(channelId5)
    defaultChannelId(channelId6)
    defaultChannelId(channelId7)
    prompt(
        OnboardingPromptType.MultipleChoice,
        title = "prompt title",
        singleSelect = false,
        required = false,
        inOnboarding = true,
    ) {
        option(title = "option 1 title") {
            channelIds += someChannelId
            description = "option 1 description"
        }
        option(title = "option 2 title") {
            roleIds += someRoleId
        }
    }
}
println(onboarding)

See discord/discord-api-docs#6101