build.gradle.kts

// In your buildSrc folder
sealed class BuildType(val secret: String) {

    object DebugBuildType : BuildType(secret = "sword")

    object ReleaseBuildType : BuildType(secret = "shield")

}

fun BuildType.getConfiguration(): BuildType {
    return when (this.name) {
        "release" -> BuildType.ReleaseBuildType
        else -> BuildType.DebugBuildType
    }
}

// In your app build.gradle.kts

buildTypes {
    getByName("debug") {
        val configuration = getConfiguration()
        manifestPlaceholders = mapOf("secret" to configuration.secret)
    }
    getByName("release") {
        val configuration = getConfiguration()
        manifestPlaceholders = mapOf("secret" to configuration.secret)
    }
}