api.video uses the RTMP protocol for livestreaming, so any tool that supports that protocol can be used.
If you just want to get started as soon as possible, here's a few that you can look at.
OBS - this is free and open source, you can use on desktop (MacOS, Windows or Linux)
Streamlabs - this is a nice app for iOS
You can also easily use our dashboard that lets you livestream with your webcam through the browser.
The code is available on Github.