In your Serverless project root directory, run:
sls module install https://github.com/eahefnawy/awsm-slack-webhook
open <serverelss project name>/back/modules/sl-slack-webhook/lib/index.js and add your callback code. This is the code that will handle the webhook.
when you're done, deploy with:
sls function deploy
sls endpoint deploy
After deployment, add the webhook url to Slack and test it out!