Steps To Do the the Poc:
1. Meta Developer Api
2. Pega Personel Edition
3. Knowledge of Rest Connectors
First Go to this Link https://developers.facebook.com/
Create an App
next screen click Business
Click next and Fill The details and create the app
You Will get the below Screen 
add your phone number to the To Dropdown which says Select a recepient phone number.
We are done with the Api setup and now we start the integration in pega.
Request payload For Message:
{
“messaging_product”: “whatsapp”,
“recipient_type”: “individual”,
“to” : “Your Mobile Number”,
“type”: “text”,
“text”:
{
“body”: “Whatapp says hello”
}
}
Request Payload For Attachment:
{
“messaging_product”: “whatsapp”,
“recipient_type”: “individual”,
“to” : “Your Mobile Number“,
“type”: “document”,
“document”:
{
“link”: “Link of your Attachment”
}
}
Here i am just goin to show the Flow and the integration activity assuming you have Connect Rest Integration Knowledge in pega
if not checkout my channel Pega for Life which has lot of integration videos on connect rest.
The case Type :
The Flow :