CRM 2013/2015/2016 Open Entity Form with Parameters

You can pass initial values for opening form using the below code. Lets open a contact form with initial value of Parent Customer

var parameters = {};
 //Set the lookup field
 parameters["parentcustomerid_id"] = "accountid";
 parameters["parentcustomerid_name"] = "accountname";
 parameters["parentcustomerid_type"] = "account"; //logicalname of entity 
 Xrm.Utility.openEntityForm("contact", null, parameters);

You need to initialize this parameters in Contact Form

You can get query string parameters in Contact Entity like below.

function OnLoad(){
var xrmObject= Xrm.Page.context.getQueryStringParameters();
var contactName= xrmObject["parentcustomerid_name"].toString();
var contactId = xrmObject["parentcustomerid_id"].toString();
var entityType = xrmObject["parentcustomerid_type"].toString();
}