Upasana Ghosh Thank you for posting your question in Microsoft Q&A. Based on my understanding, you want to setup an alert when quota limit has reached threshold in the policy.
You can define quota by subscription policy (not key) in Product scope (refer quota-policy doc) and configure Approaching subscription quota limit in the portal as described in docs: Configure notifications in the portal. The specified email recipients as well as Owners of the subscription can receive notifications when they approach 75% threshold usage. There is similar thread (https://learn.microsoft.com/en-us/answers/questions/859984/api-management-)-notification-)-approaching-subscr-notification-)-approaching-subscr)) discussion related to this with sample quota policy and feel free to check it out as well.
Please note that notifications will get triggered only based on subscription not key and also threshold is not configurable at the moment. If you like to submit feedback on this feature to our product team, feel free to do so via https://aka.ms/apimwish and others can also upvote your idea.
I hope this helps with your question and let me know if you have any questions. Please accept as "Yes" if the answer is helpful, so that it can help others in the community.