You can start a dialogue through the channel of the official WhatsApp Business API only with a template message, which is pre-moderated by WhatsApp, in detail here.
Create and send for moderation a message template in your personal cabinet.
Robot menu: Send template message
The test account contains many examples
Parameters in template from CRM
Template with buttons
Template document (media)
To send a file, you need to specify the file ID from Bitrix24.Drive, see the instructions on how to determine the file ID.