opt-in

Etymology
From.

Adjective

 * 1) The property of having to choose explicitly to join or permit something; a decision having the default option being exclusion or avoidance; used particularly with regard to mailing lists and advertising.
 * They set up an opt-in mailing list, so that those who wanted the notices could subscribe.

Translations

 * German: unter vorhergehender Einwilligung, vorhergehende Einwilligung erfordernd, vorherige, ausdrückliche Zustimmung benötigend
 * Spanish: de adhesión