Posting to a Trend (Mac & iOS)
How do I post to a trend?
A Twitter trend isn't like a message board, it's basically a search result. As people tweet, their tweets are added to all the other tweets in Twitter's database - they aren't sorted into topics. However, twitter clients, such as Twitterrific, can filter tweets according to searches (trends) or other criteria.
If you want your tweet to show up in a specific trend, consider adding a hashtag before your term. For instance many people tweeting about the Iran election included "#iranelection" in their tweet text. This allowed tweets about the event to show up whenever someone searched for "iranelection". If you had wanted a tweet of yours to show up in such a trend, you would simply include "#iranelection" in the text of the tweet.