How do I get the label added_at in a PR for a repo?

I am trying to derive some metrics from the github, for which I have created few labels in my repositories and would want to track when that is added to the PR.

I am using the below API to get the PR details but I am able to see what are all the labels have been added to the PR but unable to get the time stamp of it , How do I get the label added_at time from the github APIs ?

API route - https://api.github.com/repos/<org>/<repo>/pulls?state=closed&page=%s&per_page=100

Label section in the reponse :

"node_id":"MDU6TGFiZWwxODQ2OTA5NjQx",
"url":"https://api.github.com/repos/razorpay/fulcrum/labels/enhancement",
"name":"enhancement",
"color":"a2eeef",
"default":true,
"description":"New feature or request"
}```