Shoutbox

for 6.1.0211 it is:
Offset: 0x000206C0
Max length: 56 bytes (including null-byte)

for 6.2.0133 it is:
Offset: 0x00022A48
Max length: 56 bytes (including null-byte)

for 6.2.0137 it is:
Offset: 0x00022A48
Max length: 56 bytes (including null-byte)

So at that location you need to change "http://config.messenger.msn.com/Config/MsgrConfig.asmx" to your used URL...
And you can do this with any programming language. It's indeed just going to that location and write your own string (and terminate with a null-byte), close the file and you're done...

PS: So, I take it that your serverside script is ASMX? otherwise it wouldn't work...