The auto gene is recessive.
The only way you will get fully auto plants in a single cross will be to cross two autoflowering parents. If you cross auto and photo parents, all of their offspring will be photo. Then you will need to inbreed them to make F2. In that generation you will find 25% of plants will autoflower, and you will select from those. If you cross pollen from an F2 male(reversed female that carried the auto trait) back to a photo female from the F1 generation, the resulting seeds will be 50% auto, which may also help your selection process, but will require an extra generation to produce.