Create a new incoming webhook config via Slack app: https://api.slack.com/messaging/webhooks (or https://slack.com/apps/A0F7XDUAZ-incoming-webhooks which is easier but apparently being deprecated)
Example .dotscience.yml configuration:
kind: pipeline
after:
- name: circleci
image: quay.io/dotmesh/dotscience-slack-plugin
settings:
slackUrl: https://hooks.slack.com/services/.../...