I need a JS script for Chrome extension which searches certain text in the HTML code of the page.

YouTube page of the channel contains channel id in the source code: "channelId":"UCsNGtpqGsyw0U6qEG-WHadA"

The script should find this peace of text and return only channel ID: UCsNGtpqGsyw0U6qEG-WHadA

How can I do it?