Install package

Select Window/Package Manager in the menu bar.

Install Package Manager from menu bar

Check Package Manager window, Click + button and select Add package from git URL....

Select add package from git url

Input the string below to the input field.

com.unity.webrtc@3.0.0-pre.8

The list of version string is here. In most cases, the latest version is recommended to use.

Input webrtc package git URL

Click Add button, and will start install the package.