Update: We've recently added new features to the platform that are not shown in this video.
Once you've clicked into the assessment of your choice, you will now be able to customize emails sent to candidates by navigating to the Invite tab. Once the invite tab is open, click into the Invite Settings section, and then the "Edit and Preview Email" button on the top right side. This will allow editing of email invitation sent.
To edit the rejection email, scroll down until you see the "Preview and edit rejection email" link on the right side in the middle of the page.
For more information on the emailing feature, please refer to this article.