The M8 has no flash TTL, like any other sensor camera. The sensor is too shiny to allow true flash TTL. There is M-TTL (comparable to Canon's E-TTL, but not the same). The only flashes that can use M-TTL are the SF24D and a couple of the newest Metz flashes with SCA 3502-M5 adapter. All other flashes must be used in automatic mode.
Metz flashes on SCA 3502-M3 through M5 will set shutter speed and give flash-ready in the viewfinder.
Having said that, the best flash results I have ever seen from an on- camera flash are with the M8 and an old Metz M2 or MZ3 flash om automatic. Far better than for instance the DMR/R9 combo I am getting to grips with. And also in another class from the SF24D, even with SFill soft-box.
The M8 has a high-voltage resistant flash sync. Still, with a doubtful flash I would use the Wein safe-sync.
M8, Summicron 35/2.0 asph @ 5.6, ISO160, Metz Z2, set to ISO 200, middle "A", reflector @ 50 mm and 45 degrees tilt.