To obtain the channel id you can view the source code of the channel page and find either data-channel-external-id="UCjXfkj5iapKHJrhYfAF9ZGg"
or "externalId":"UCjXfkj5iapKHJrhYfAF9ZGg"
.
UCjXfkj5iapKHJrhYfAF9ZGg
will be the channel ID you are looking for.
https://commentpicker.com/youtube-channel-id.php
Given a particular channel URL: https://www.youtube.com/channel/UC9mPk8yIk1AUC8PDsUYFBaA
You can fiddle a little bit and: