Is the “away” mode of Hue a scene? Then you should be able to activate it quite easily via the Service “HUE.ACTIVATE_SCENE”.
Otherwise you maybe want to have a look at this: Holiday / Vacation mode - Configuration - Home Assistant Community (home-assistant.io) 
Or even better, you can just use this integration from the HACS Store: slashback100/presence_simulation: Home Assistant Presence Simulation (github.com)