All you have to do is create a suitably sized array of characters - 26 to hold
including the terminating newline and null, then copy in three parts using strcpy:
AT+CMGS=" - A fixed string
973898045674 - the number to send it to from your struct
"\n - the terminating close quote and the "GO" instruction
The modem will then send back four characters as a prompt ready for the message body.