Hi, @Tomer_Klein Before I found this topic I also was working to find a Whatsapp self-served solution in the HA Docker environment. I found my solution in the rest_command (and using https://github.com/chrishubert/whatsapp-api):
rest_command:
send_whatsapp_web_api_string:
url: "http://localhost:3000/client/sendMessage/HASS"
method: post
headers:
content-type: application/json
payload: >
{
"chatId": "<see_contacts>@g.us",
"contentType": "string",
"content": "Keldervloer - *{{ states('binary_sensor.waterleak_water_leak') }}*"
}
f.y.i Conditional send inside an automation
To be honest I had/have some problems to find the right syntax because examples are scares but luckily one issue mentioned the required URL sendMessage-string 
. And with some inspiration of this topic you can see that above example also sends the value of the sensor (perhaps a helpful addition).
I am still looking for the example how to retrieve an incoming message it would be most welcome. Think the method will be likely “get” but what is the URL??? If you (or someone else) already knows how to do this please share.