While the default key combination to target your companion is Shift + F1, you might also be interested in learning about the focus system so you can quickly heal primary healing targets in many situations.
A focus is like a secondary target. To set focus, first target something, then press Alt-F. Now you may target something else. If you want to switch your target and your focus, press Alt-F again (trivia, not how this is normally used).
Now you have a focus, but to use hotkeys on it you must go to:
preferences -> controls -> enable focus target
Once you've done that, you may use the focus-modifier-key (
end by default) to use abilities on your focus instead of your target.