The calendar is automatically linked to my iCloud account, which is not the calendar in use – How can I choose my preferred calendar to sync with? Appreciate the help in advance!
In order for Agenda to see the calendars you want to use it’s important that they first show in the system calendar app. This means you will need to configure the relevant calendar accounts in the system preferences (macOS) and settings app (iOS+iPadOS). Once the calendars show in the Calendar app, they should also appear in Agenda.
In Agenda you can limit which calendars to show in the Agenda preferences (premium feature) if you like. Also, if you create a new event from within Agenda (also a premium feature), you can select the calendar on which it should go. Agenda should then remember that choice as the default.
Hope that helps, see also Dates, Events, and Reminders