I have a couple of questions for the OP-
I'm assuming you show in QH breed shows, is that correct?
And you want to show your horse in the hunter divisions, not just the WP? And your horse is confirmed in the WP gaits; and when you ask her to to a huntery, English style, forward trot, you run into problems?
If that's correct, what you want is lengthening, not extension, and the correct way to ask is with alternate leg aid. "Extension" is often understood as a dressage term for a fairly advanced movement that requires collection.
RiderGirl's advice is based on dressage training, and while correct to ask for true extension in the dressge sense, I think you're looking for something different.
Alternate leg aid means applying first one leg, then the other, in rhythm with the gait. For your horse, it may be easier to teach at the walk, using alternate leg aid to ask her to lengthen her walk for a few strides at a time, then letting her relax back to her normal walk. If she masters that, posting trot, and down the long sides of the arena, ask her to lengthen her trot by using inside leg as you rise, outside leg as you sit. Rhythm is more important that the strength of the aid, little squeezes or taps are plenty. If she breaks, gently bring her back and ask again on the next long side, or go back to your lenthening the walk exercise a few times before asking for a lenthened trot again.