How to Integrate Slack & WhatsApp on Linux Mint 20 Guide
How to Install WhatsApp and Slack desktop Chat Messenger Client in One App on GNU/Linux Mint 20.x Ulyana/Ulyssa/Uma/Una desktop – Step by step Tutorial.
And to Install WhatsApp & Slack Client on Linux Mint 20 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 Linux Mint 20 Installation Guide