Creating a rectangle mask in Premiere CS6

hey there! in video i've got mock image of tablet, , want text on mock image swipe off left of screen.

so want create rectangle mask on screen, letters/words disappear text swipes left off screen. i've searched solution haven't had luck far. best way of achieving this?

 

any appreciated. thanks!

if mean this, create new color matte white color , mask out tablet screen size effect control window

 

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