The Subscription confirmed approval page will be shown to the user if the list to which he is subscribing has authentication enabled.

Step 1: Once into the Lists section, click on the Manage button on the Pages Tab.
Step 2: Click on the Select another list page to edit dropdown button.
Step 3: Select the Subscription confirmed approval option from the List.
Step 4: Click on the Source button and edit the message as per your needs.
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