I made a post about this earlier in the SOD forums, but it seems to have been deleted without any notification (website bug?).
Settings → Options → Gameplay → Combat → Focus Cast Key allows us to set a key which will be used to automatically target spells at our focus target.
However, in SOD, there doesn’t seem to be a way to set a focus target. /focus doesn’t work, nor does setting it via a macro. Right click doesn’t give us an option to set a focus target.