Short answer
HubSpot's KB: once a company or deal is linked to a Slack channel, new calls, notes, emails, tasks and meetings logged on it trigger a Slack notification. Use the Slack card in the right sidebar, or a workflow to create and associate channels.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Before merging records, associate the channel with the primary record, per the KB.