If you already have the integration with Indeed enabled, you just need to go to Settings --> Advertisement and clauses --> Advertisement and move the slider called "Activate Indeed Apply service" to the right:
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If you didn't have the integration with Indeed activated yet, you need to go to Settings --> Advertisement and clauses --> Advertisement and move the slider called: "Send all advertisements published on your Career page to Indeed automatically" to the right. When turning on the integration with Indeed, Indeed Apply will become enabled automatically.

You can disable Indeed Apply anytime - you can check out here how to do it.

Integration with Indeed is required for enabling Indeed Apply functionality.