no search result with URL in tweet

When I post a tweet "first message", my tweet appears in search results (searching on "first message"). When I post a tweet "first message http://my.url.com", the tweet does not appear in search results.

How can I use an URL in my tweet which appears in search results? I know it is possible, because I see tweets of other people in search results.

Cheers,
Koekie

 

Put # hash tag before your keyword