1
Move your cursor to the desired position on the timeline where you want to
add a rectangle, then navigate to the Elements option on the left menu.

2
Click on Rectangle. 

3
Adjust the position of the newly added rectangle. 

4
Click on the element to access the rectangle settings. Modify the stroke
color, stroke width, fill color, border radius, and the opacity of the fill
color. 

5
In the Animation section, choose the animation for the rectangle.

6
Preview the rectangle you’ve just added.