You can send flash sms aka class 0 ONLY if your device allow it.
Sending flash SMS require binary PDU mode. In PDU you can encode in 7 bit.
Of course in PDU mode you can send flash and normal SMS as well. But for  the network class 0 is accepted only in binary PDU, see specification  of standard GSM 03.38
All Nokia (non smartphones and smartphones) implement PDU mode. Some  samsung models also, motorola dont, and sony I think don't. I don't know  why but some manufacturers deliberately don't implement this part of  the standard because preventing abuse. Flash SMS are low level class 0  network message and they don't like users to have control
So it's up to your device you can see this with this command AT+CMGF? If  it answer 1 means your phone cannot send Flash SMS because PDU format  is forbidden. For example you check free flash SMS without registration  at www. cybercom. com.au There are many others providers but especially for flash SMS they are paid 
There is many tutorials how to check your phone, just google for  AT+CMGF? and if your phone answer 0 than you can send flash SMS.  Otherwise whatever app you try it will not work cause hardware don't  accept it, coz not software issue