Здесь же все подробно описано:
Send Message
Resource URL:
https://chatapi.viber.com/pa/send_message
Post data:
{
"auth_token": "4453b6ac1s345678-e02c5f12174805f9-daec9cbb5448c51r",
"receiver": "01234567890A=",
"sender":
{
"name": "yarden from the pa",
"avatar": "
avatar_url"
},
"tracking_data": "tracking data",
"type": "text",
"text": "a message from pa"
}