_shopmsg('visitor.set', {"propertyName" : "propertyValue"});
To set properties about a visitor, call the visitor.set function. You should set things like first_name, last_name, or email if you know about them when the visitor lands on your site.
// set whatever properties you want about a visitor
// you can use these attributes in your flows or segmentation
_shopmsg('visitor.set', {
"first_name" : "Jane",
"last_name" : "Smith",
"email" : "ja[email protected]",
"customer_id" : 12379834
});