This is possible in customized training mode, under "Options" in the exercise setup. In Tutor mode, the notes played before the questions (cadence, root chord or key tones/tonic) depend on how each lesson is set up.
i can choose the tonic of the scale but i cant figure out how to make it play Tonic always on the first beat
I press F3 in customized training and it still dosent let me do it
Sorry, I think I misunderstood your question. I thought you were referring to what is played before the question, not to the first note of each question. It is unfortunately not possible to force EarMaster to play the tonic as the first note of each melody.