How do I do this? I have a Paladin and would like to assign Divine Might to a hotkey instead of going through the long option tree to invoke it.

There's a quickbar at the bottom of your screen. When you hold Shift or Ctrl, it changes to show even more empty fields. Each of the fields is linked to one of the 12 F keys at the top of your keyboard.

You can drag and drop anything from spellbook spells to inventory items onto the quickbar, and even set dual-wielding or sword and shield styles by stacking suitable weapons and shields.
You can also add memorised spells, abilities and many other things to the quickbar by right-clicking an empty slot. This will bring up the radial menu (your 'option tree'). Select the property you want to use (Divine Might) and you're done.
From then on, whenever you want to use the ability you set in the slot, just use the corresponding hotkey.

If you want to change the hotkey, you'll have to delete the previously assigned hotkey then reassign a new one.

This hotkey business is important for RP gamers...emotes!

bow, greet"hi!", drink "get drunk", read "writing swims due to drunkedness", fall backwards "drunk", scratch head "wondering what happened", threaten "the nearest halfling for knocking you down", beg "when you realise that that halfling is level forty with level draining greatsword", worship "to make halfling happy" then finally goodbye "run away!"