It seems like the designer does not support Kotlin yet. Here are some solution:
XML (Not Recommended)
Add the following line to your Button tag. This is exactly what the designer will do.
android:onClick="sendMessage"
Old Fashion
No need to add anything.
val button = findViewById<Button>(R.id.Button)
button.setOnClickListener {
}
kotlin-android-extensions (Recommended)
Add apply plugin: "kotlin-android-extensions" to your build.gradle
// button is the Button id
button.setOnClickListener {
}