You can't without their permission, there is a method if you get them to accept a file you're sending, but this is illegal, and cannot be discussed here sorry.
If they have messenger plus!, ask them to say (!IP)
(to test if they have messenger plus! type in /ping and press enter in the conversation window, you will automacally say
[Ping!] and they should reply with something like
[Pong! 02secs] if they have messenger plus!)
