Probe is built around a small, memorable shortcut set. Most actions are reachable from the menu bar; on macOS the shortcuts here mirror what you see there.
File → Save Session… (⌘ S on macOS) saves what's in front, on macOS and Windows alike: in a Composer tab it saves the request (there the item reads Save Request…), in a session tab with unsaved traffic it opens the Save Session dialog, and in a saved session tab, or the Ignored, Files, flow, WebSocket or History tab, it does nothing.
The row's right-click menu shows keys next to Compose (⌘ E), Copy URL (⌘ C), Copy as cURL (⌥ ⌘ C), Add to Favorites (⌘ D), Save to Collection (⌥ ⌘ S) and Ignore (⌘ I), with Ctrl for ⌘ and Alt for ⌥ on Windows. These are labels only for now: pick the action from the menu. They stay clear of the menu bar's keys, which win: ⌘ F opens Super Search, ⌘ S saves the session and ⇧ ⌘ C toggles the Probe Console on macOS.
Ctrl S is File → Save Request… (the File menu's Save item while a Composer tab is in front), which saves that request (Save to Collection for a new request). Sending has no Windows shortcut yet: use the Send button next to the URL.
While typing inside the URL or body, the {{ trigger opens the variable autocomplete. Inside the popup: