I don't have the best grasp on the intricacies of afterburners but I'd assume that if the engine has a flame holder then fuel would be injected before it in order to allow it to heat up to autoignition by the time it reaches the flame holder. If not please explain, thanks.
This is what I could find on the matter in my university textbook.
The fuel spray bar is typically composed of one or more rings with distinct fuel injection heads circumferentially distributed around the ring. The V-shaped flame holder ring(s) create a fuel–air mixture recirculation region in its turbulent wake, which allows for a stable flame front to be established.
Although it does not mention fuel heating explicitly, I would agree with your assumption -- as the mixing of the hot gas and the (colder) fuel of course also evens out temperature differences between the two.
Source: Farokhi (2014), Aircraft Propulsion (p. 199)