Sendspark has countdown timers for both the Chrome extension and web application. Here is how to use both:
Countdown in Chrome Extension
Camera & Screen Recording
Web Application
Camera Recording
Select Camera recording to launch the recorder
Click the countdown timer on the bottom left of the video player to toggle it on/off
Screen Recording
There is no way to toggle on/off the countdown timer for screen recording, but you can skip countdown to start recording sooner — so you don't have to wait!