From the web application

Navigate to your incident page and click on the Roles tab. Next to your role, click on the assignee dropdown, search for the on-call schedule from which you want to assign the role. Zenduty will fetch the first person who is on-call from the schedule and assign them to the role.

From the Slack/Teams bot application

Within the incident message, inside the role section click on the … button and click on Assign Role. Search for the on-call schedule from which you want to assign the role. Zenduty will fetch the first person who is on-call from the schedule and assign them to the role.