On a video: open the Matinee panel and press Save. It goes to your Inbox; nothing to choose. The line then reads In Inbox (or the folder's name), with Edit.
Any YouTube link, anywhere: right-click it → Save to Matinee.
Keyboard:Alt+Shift+S saves the video you're watching (change it at chrome://extensions/shortcuts).
Paste a link into the search box and press Save this link.
Drag and drop: drag a video's thumbnail or title from any page onto the panel, and drop it on Save to Inbox, Play now, or straight into a folder.
On YouTube's own pages, Save to Matinee also appears in the ⋮ menu of videos, and a Matinee button sits under the player (Settings can hide these).