When you run a Shortcut from Services it can use the current selection, be that text or image or whatever, as input -- if the app supports such. Assuming you are still trying to script Bear, if you've some text selected then that will be the "input from Quick Actions", and you can limit what it can receive (and, by extension, when the shortcut will be available) by changing "Any" to eg "Text and Rich Text".
As I (very vaguely) understand it, it's the app making the selection available to Quick Actions. Which does suggest we could use a Quick Action-enabled Shortcut from a KM action to do the same.
Except already-selected items are generally available to KM -- via the "Copy" command and the Clipboard. The difficulty is usually "I want to right-click the selected text" or similar, and we're not doing that here because we've worked round it.
I've heard good things about it -- and I look forward to your dazzling KM/Shortcut integrations!