For users already familiar with CommCare's conditional select functionality, referencing locations within a form is very similar. To access the list of locations from within a CommCare Supply form, you can use the syntax described HERE.
Much like lookup tables in order to reference location data in a form, you must first include a reference to this instance in the forms xml. To access this xml, enter your form, and in the top righthand corner, under "Tools", select "Edit Source XML"
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Add the feature preview "Custom Single and Multiple Answer Questions" to your project