Send custom SMS for wifi guest accounts

Hi,

I want to set up wifi guest accounts password via SMS services. I know that we can buy some SMS services through FortiGuard but I want to use a custom server (bulksms services) as we already have an account there.

Problem is I need to have a specific from (an authorised email address) and subject (password for bulksms). In global I have find : 

config system sms-server
    edit "bulksms"
        set mail-server "bulksms.net"
    next
end

But no way to fix the from and subject. Is there a way to do it ?

Regards