PrEvent:
    | PRCommentAdded
    | PRCommentDeleted
    | PRCommentEdited
    | PRDeclined
    | PRDeleted
    | PRFromRefUpdated
    | PRMerged
    | PRModified
    | PROpened
    | PRReviewerApproved
    | PRReviewerChangesRequested
    | PRReviewerUnapproved
    | PRReviewerUpdated

You can create webhooks for events that occur on a pull request.