Homepage Search – All Location changes

Hi All,

I am using the September, 2016 child theme and would like to link the “All Locations” search dropdown field to link to a custom field added to listings. This way users can carry out a search that is relevant to the listing and it’s custom field using either a drop down pulling from the database field or an input field that looks at entries in the custom field of the database.

Any help with this is very appreciated.