What do you need to do before you can define a custom pattern for a...

Comprehensive and Detailed Explanation:

Before defining a custom pattern for secret scanning in a repository, you must enable secretscanning for that repository. Secret scanning must be active to utilize custom patterns, which allow you to define specific formats (using regular expressions) for secrets unique to your organization.​

Once secret scanning is enabled, you can add custom patterns to detect and prevent the exposure of sensitive information tailored to your needs.​