There's no slang substitute without looking stupid. You could write 'wtchng' but I do not recommend unless you are on a pay-by-character plan.
gtk
Text You Back
make love to me
killing myself laughing
First, look up some slang, if you don't already know any. Most people know lots of slang. Second, imagine two people talking. Third, write down what you think they would say in slang.
welcome back (to a text or chatroom) or write back lyk tb text back
penis
. On My Way
lpl
Miss You!
IZZY
gtk
Text You Back
make love to me
Before You Leave
No big Deal!
Did not laugh/does not like