How to generate decryption key from m3u8 manifest? (OR) How to decrypt an excrypted MP4 using URI and IV?
You cant generate the key. You have to download the key. The URL is in the manifest. In this case it looks like you need to substitute some values first. 

To decrypt, since this files uses AES-128 and not SAMPLE-AES, you can just decrypt each file using OpenSSL