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When you sign up for an Azure Free Trial, you must enter your credit card information, but these are only used to register you and will not be used to charge you. Once the trial period is over and the available credits are exhausted, you can decide whether to switch to a pay as you go solution, or if you decide not to proceed, you will no longer be able to use Azure services, and you will not be charged. At that point, you can decide to close the tenant and delete the tenant.
This is the link for activate your free trial
Hope this helps
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