Set value on newly created node using Rules
In the Rules action you need to use the Set a data value action. This can be used to enter data in fields and properties. Make sure to set the value of your new 123-entity to refer to the xyz entity. 

{ "rules_create_new_basic_page_after_saving_article" : {
    "LABEL" : "Create new basic page after saving article",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_insert--article" : { "bundle" : "article" } },
    "DO" : [
      { "entity_create" : {
          "USING" : {
            "type" : "node",
            "param_type" : "page",
            "param_title" : "Basic page to go with [node:title]",
            "param_author" : [ "node:author" ]
          },
          "PROVIDE" : { "entity_created" : { "page_created" : "Created basic page" } }
        }
      },
      { "data_set" : {
          "data" : [ "page-created:field-reference-to-other-entity-" ],
          "value" : [ "node" ]
        }
      },
      { "entity_save" : { "data" : [ "page-created" ], "immediate" : "1" } }
    ]
  }
}