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First thing, you need to select value of "Events Source" and click "Select". Please note that here for "Mixed" calendars you can select from internal events sources. You can not add an event to sources that have type "iCal" or Jira dates"Mixed" calendars allow events creation for "Jira dates" sources and this results into adding of Jira issues.
Some event types require you to enter users, project or issue. Please make sure to fill all the required fields.
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