Click the details arrow next to the field that you want to turn on notifications for.

Click the "Notify task collaborators when this field's value is changed" field.

Click "Save changes". Notifications for that field in that project are now turned on.

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This guide provides step-by-step instructions on how to turn on custom notifications for a specific field in Asana. By following these steps, users can ensure that collaborators are notified whenever the value of a field is changed, improving communication and keeping everyone updated in the project.