From what I can tell, it looks like it'll be an M12 1.25 bolt. Get it in the highest grade stainless metal you can find. I would probably recommend no coating on it if possible (I have some high grade stainless stuff that the coating flaked off on the rest of my exhaust).

Get a nut that will go on it, and if you can, check the length so it doesn't protrude into the exhaust stream. Use the nut to fasten it down at the required length (or just cut part of the threads off to length). Use some high temp thread locker if you have it. Maybe a crush-ring for extra sealing protection.