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" } }
]
}
}