我想寄信比 jesse mccartney...

I trhink it's hard to do what you want
cuz usually they won't do this
You can see him and ask for his sign
But since you are in HK, you can't
Then I suggest you to go to
First,
http://www.jessemac.com/ to see if there's any fanpage and leave a comment

Second , go to his facebook
http://www.facebook.com/JesseMcCartney
add him and leave a comment or send a message to him

then to see if he can full fill your dream

or you can ask your friends who are in western countries help you to get his sign

or you can see and wait if he will come to HK / When will he come to HK ?tnen , you can ask for his sign.