Access_key can be obtained using the newsfeed.get method in the parameters by specifying filters = post and count = 1 . The response will be in addition to information about the post, access_key .
Sample answer:
{ "response": { "items": [ { "type": "post", "source_id": -57846937, "date": 1467650412, "post_id": 2809380, "post_type": "post", "text": "", "attachment": { "type": "photo", "photo": { "pid": 418453269, "aid": -7, "owner_id": -57846937, "user_id": 100, "src": "http://cs7005.vk.me/v7005482/2a266/ykQqTloRWy8.jpg", "src_big": "http://cs7005.vk.me/v7005482/2a267/2O00C6MUGxc.jpg", "src_small": "http://cs7005.vk.me/v7005482/2a265/Aq7yK_-gQ_U.jpg", "width": 480, "height": 423, "text": "", "created": 1467648178, "post_id": 2809380, "access_key": "18d3696d47dd4867a3" <== KEY } }, // Остальной код }