How to Integrate Slack & WhatsApp on Kali Linux Guide
How to Install WhatsApp and Slack desktop Chat Messenger Client in One App in Kali GNU/Linux.
And to Install WhatsApp & Slack Client on Kali Linux you dispose of some cool Alternatives:
Especially relevant: currently Ferdi is the only completely Free alternative, and so I listed them in order of my Personal Testing Preference.

WhatsApp & Slack Client Kali Linux Installation Guide