I'm using GSConnect on Ubuntu. I have first disabled "Clipboard Sync" to avoid it transferring every clipboard content. Secondly, there are a number of actions for which keyboard shortcuts can be defined for a device [1]. One of them is Clipboard Push. I assigned Shift+Ctrl+Alt+V to that, so whenever I execute that contrived key combination, the clipboard is synced to my mobile. I suppose the Plasma implementation has a similar feature.