Custom URLs modify your tracking links to match your organization's branding. After sending an email through CM, tracking links are appended to the front of any URL in the email. It looks like {https://contactmonkey.com/api/v1/tracker?cm_session=}, with a long session ID to follow.
By updating your custom URL, we are modifying this tracking link to instead look like {https://custom.yourdomain.com/api/v1/tracker?cm_session=}. This helps to personalize the experience within your organization and supports keeping links internal in your email host.