I found it out.
During a call:
I have to press the Flash button first,
Waiting for the dial tone,
And then press *98 ( optionally followed by the "#" key, to speed up the process ),
And then wait for a slightly different dial tone,
And then dial the the target number ( optionally followed by the "#" key, to speed up the process ),
After I hear a 3-beep confirmation tone, I can safely hang up my phone, because the blind transfer is now occurring in the background.