How to Integrate Slack & WhatsApp on Ubuntu 18.04 Guide
How to Install WhatsApp and Slack desktop Chat Messenger Client in One App on Ubuntu 18.04 Bionic LTS GNU/Linux – Step by step Tutorial.
And to Install WhatsApp & Slack Client on Ubuntu 18.04 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 Ubuntu 18.04 Installation Guide