Cannot align Facebook audience banner to the bottom

I'm Facing a problem with adView.Show(100); cannot align it to the bottom of the screen. I tries put inside UI Canves element align to the bottom and make adView.Show(0); but it's align to the top of the screen not of the parent element. tried also adView.Show(canves.rect.height - 50); it's appear on my phone screen good but on other screen totally in wrong position.

any help.