Use whatever mode to meter the sky, get a reading, let says f5.6, 1/125s, so to make the sky come out nicer, you need to underexpose it by 1 stop, so you can set you camera at f5.6, 1/250s, M mode and you flash TTL, for this the b/g is underexpose by 1 sotp, and the subject will expose by the flash according the "correct" exposure, but you need to adjust the camera setting and the flash compensation for fine tuning the final result.