The User Data Header of SMS [0] isn't very flexible, and quite constrained - both it and the message needs to fit inside a 140 byte payload.
There are a handful of bytes reserved for a future purpose, which could be used for something like this, but you're limiting how large the message can be, likely significantly.
There are a handful of bytes reserved for a future purpose, which could be used for something like this, but you're limiting how large the message can be, likely significantly.
[0] https://en.wikipedia.org/wiki/User_Data_Header