Salesforce's default security settings require you to whitelist any external domain that is called by an application.
By default, the package whitelists webhook connectors for the following services: Slack, Zapier.
Setting Up a Custom Site
Navigate to Salesforce Admin.
In the quick nav search 'Remote Site'
Click on 'Remote Site Settings'
Click 'Create New Site'.
Fill out the site settings.
NOTE: Salesforce requires you to whitelist sites at the subdomain level, so if you are going to call 'hooks.slack.com', whitelisting 'slack.com' will not work.
