Until the dû becomes well known enough to get it’s own key on the keyboard (we hope by 2020!) we’ll have to use the existing keyboard strokes.
Iphone, iPad and mac users
Simply hold your finger on the u key for a second, notice that a number of options, including û now pop. Choose it and use it
Mac users ( 2nd option)
Simply press and hold the alt/option key while pressing the i key then release both and press the u to get your perfect û
alt + i