To quickly add a term select source and target and press
Ctrl+Shift+F2. The downside is that it is not that quick as the Termbase Viewer Window has to appear (when windows are set up with the default way), slowing down the workflow until you hide it/close it or it autohides.
What one can do is change the layout to something like the screenshot. To move windows around see:
Moving windows… | multifarious