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 or
www.cybercomsms.com 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