To get this effect, you need a good amount of gap clip before the first video clip. Before the first clip, choose 'Insert Gap' from the Edit menu.
In the Transitions Browser, choose the Wipes category.
Drag the Center effect between the gap and the first video clip.
The Center effect defaults to feathering the edge, so to make it hard, drag the little diamond to the left until it touches the arrow.
To make the transition vertical, drag the arrow around the centre point until it is vertical.
To make the transition slower, in the timeline drag the left or right edge of the transition to make it wider. Avoid the top left or right of the transition, that control is for rippling the clips around the edit.
If you can't make the transition wider, you don't have enough source material in your first video clip; you'll need to drag the centre of the top-centre of the transition to the right before making the transition wider.