Hey!
I was on the same thing.
Finally I managed adding ?via=USERNAME after (replace USERNAME for your twitter username, without @)
twitter.com/intent/tweet
so it might look like
twitter.com/intent/tweet?via=USERNAME
the entire line is
$twitter_url = add_query_arg(
urlencode_deep( array_filter( compact( 'via', 'related', 'text', 'url' ) ) ),
sprintf( '%s://twitter.com/intent/tweet?via=USERNAME', ( is_ssl() ? 'https' : 'http' ) )
);
It works!