Final answer:
To onboard a large volume of external users to a custom application, the 'external user self-registration' plugin should be activated.
Step-by-step explanation:
The plugin that needs to be activated to onboard a large volume of external users to a custom application in your instance is a. external user self-registration (com.snc.external_user_self_registration). This plugin enables external users to self-register in the system, making it ideal for cases where there is a need to facilitate the onboarding process without manual intervention for each user. For example, if a university wants to provide access to a custom application to all incoming students, the self-registration plugin would allow students to create their accounts independently.