There is a "Copy channel ID" link on each channel's page, but it's well hidden. Click "...more" in the channel description, then click "Share channel" to open a

There is a "Copy channel ID" link on each channel's page, but it's well hidden. Click "...more" in the channel description, then click "Share channel" to open a popup menu that has the "Copy channel ID" link. It does what it says.