In the applicant portal configuration the following line needs to be added.
<!-- Redis cache configuration-->
<add key="CacheEndpoint" value="REDIS_CONNECTION_STRING" />
After the configruation of the applicant portal has been completed, double-opt-in needs to be activated for an applicant portal in the eRecruiter settings.
After activating this option, double-opt-in should be activated for registration on the applicant portal.