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Ensure you're added to your school's test Campus Cloud and test user environment (email your Campus Success Consultant if you need access)
Switch your campus mobile app from Production (live environment) to Test. Here is an article for your reference: Settings for Test Environment
Create your message under the Notify tab in the test school on Campus Cloud, select Everyone as your audience, and the type of announcement to Emergency
All users with access to the test app environment in your test environment should receive this test emergency notification. See how to invite users to the test environment here.