The whitelist feature allows for users to keep certain retweets on their profile permanently so that they do not get deleted at every unretweet cycle.
To whitelist a retweet will require the tweet’s ID. You can find the tweet ID by clicking on the tweet and it will be the sequence of numbers at the end of the web address (shown above). Make sure that at this point you have not retweeted the tweet yet. Now copy the 18-digit number, sign in on spamcleaner.co website and paste it into the given box labeled insert tweet id (shown below).
After you paste the number, click on “add to retweet whitelist.” Now you can go back to twitter to retweet that tweet and it will never be unretweeted from your timeline.