From the All Items screen, navigate to Releases on the left-hand side and select a Version you'd like to set-up post-market vulnerability notifications.

Then navigate to the Release's Settings, scroll down to the Monitoring section and select Active. 

Notes:

1. Post-market vulnerability notifications will only be sent via email once the Version where this setting is set on Active has been released. 
2. When Automatic (default) is selected, only the release with the highest version number according to Semantic Versioning is considered as currently active and will be included in the notifications.