Hi,
SWD is interface on the microcontroller used for programming and debugging. OpenSDA is also "SWD programmer/debugger".
If you have blank chip, the only option is to use OpenSDA (or any other tool using SWD interface) to program the chip. There's nothing like in-built bootloader or something like that.
If you want to use Flash API to program the chip, you would have to load some code via SWD anyway (to RAM, for example) but I can't see an advantage of doing this. That's just unnecessary step.
Regards,
Lukas