Get all repos on CircleCI
curl "https://circleci.com/api/v1.1/projects?circle-token=$CIRCLE_TOKEN" | jq -r '.[] | .username + "/" + .reponame'
Set slack_webhook_url
for a single repo
curl -XPUT --header "Content-Type: application/json" "https://circleci.com/api/v1.1/project/github/MyOrg/MyRepo/settings?circle-token=$CIRCLE_TOKEN" -d '{"slack_webhook_url":"https://hooks.slack.com/services/FSFDSFDS?DSFHJKHDFKJSDH"}'
Put it all together to setup webhooks for all of your repos