What do you touch for the Forbiddance spell?

Touch spells typically either specify the target that you touch (i.e. Magic Weapon) or involve a spell attack where the rules of targeting are governed by the chapter on spellcasting:

Some spells can target only a creature (including you) that you touch.

Forbiddance doesn’t specify what you touch or whether you must. How do I cast this spell? What do I target?

Range touch […] You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the┬áDuration, creatures can’t┬áteleport into the area or use portals…