1
Move your cursor to the desired position on the timeline where you want to
add a callout, then navigate to the Elements option on the left menu.
2
Click on Callout. 
3
Adjust the position of the callout rectangle that appears.
4
Click on the element to access the callout settings. Modify the stroke
color, stroke width, background color, border radius, and scale factor as
per your requirements. 
The scale factor preview updates only while the video is playing. Changes won’t appear when the video is paused.
5
In the animation section, adjust the draw time and transition time.