There is 2 ways you could do it
Either
have it click to activate a move module, with a variable set to thrust and the afterburning plume elements, and only staged to activate the engine itself.
This one is more complicated, and doesn't let you activate the engine by clicking, but it allows you to have more control over the engine output.
Or alternatively
You could have it in the plume throttle move module and for it to activate after a certain throttle value, of which the thrust will increase, and the afterburning plume element would show up.
This one is simpler but, doesn't let you do low throttle with afterburner, nor does it let you do high throttle without afterburner