app/assets/javascripts/spree/backend/checkouts/edit.js in spree_backend-3.0.0.rc3 vs app/assets/javascripts/spree/backend/checkouts/edit.js in spree_backend-3.0.0.rc4

@@ -17,10 +17,13 @@
       placeholder: Spree.translations.choose_a_customer,
       ajax: {
         url: Spree.routes.user_search,
         datatype: 'json',
         data: function(term, page) {
-          return { q: term }
+          return {
+            q: term,
+            token: Spree.api_key
+          }
         },
         results: function(data, page) {
           return { results: data }
         }
       },