There are two ways to begin this mission. You will receive the information about attack during the 21st main mission (The War Economy) and you are going to face a decision presented on the screenshot above. If you want will press the "OK" button and react immediately you will interrupt the 21st mission. If you will ignore this option ("Cancel") you will be informed that the Strike Team has succeeded to secure your Mother Base however they were unable to safe any hostages. It doesn't mean that you will not be able to complete the 22nd mission on your own. You just need to select it the same way as you choose any other mission in the adequate menu.
Remember - Later on you will receive these kind of messages when you will be attacked by other players. You may read more about it in our second guide to the MGS V.