Send "welcome message" with keyboard to Viber bot

Viber API allows to send some message on conversation_started event type to allow user to subscribe. From documentation about "welcome message", I see following code, that successfully sends text and image:

{
    "sender": {
        "name": "John McClane",
        "avatar": "http://avatar.example.com"
    },
    "tracking_data": "tracking data",
    "type": "text",
    "text": "Welcome to our bot!",
    "media": "http://www.images.com/img.jpg",
    "thumbnail": "http://www.images.com/thumb.jpg"
}

But how to add some buttons there?
I want my users to be able to press them to subscribe and start conversation with my bot.

I tried to add following to message, but it didn't work:

"keyboard": {
    "Type": "keyboard",
    "DefaultHeight": true,
    "Buttons": [{
        "ActionType": "reply",
        "ActionBody": "reply to me",
        "Text": "Key text",
        "TextSize": "regular"
    }]
}

After some tries I figured out that can't use both media + thumbnail and keyboard in the same "Welcome message". So I removed media and thumbnail keys. Now the following code works:

{
    "sender": {
        "name": "John McClane",
        "avatar": "http://avatar.example.com"
    },
    "tracking_data": "tracking data",
    "type": "text",
    "text": "Welcome to our bot!",
    "keyboard": {
        "Type": "keyboard",
        "DefaultHeight": true,
        "Buttons": [{
            "ActionType": "reply",
            "ActionBody": "reply to me",
            "Text": "Key text",
            "TextSize": "regular"
        }]
    }
}