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Go to Setup > Profile > Standard Platform User / Standard Platform One App User > Object Settings > Internal Use My Calendar > Default On > Save.
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Note: You need to configure the Profile of the User that you want to give access to the Internal User My Calendar.
In the Standard Platform One App User profile, go to the Apex Class Access and the following Apex Classes
PosiEd.CalendarController
PosiEd.ColorHexGenerator
PosiEd.CustomCalendarPortalHandler
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You can now check the Internal User My Calendar by searching it on the App Launcher.
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