Below is the code I am using:
//Set the reqarding field value
parameters["pId"] = "2878282E-94D6-E111-9B1D-00155D9D700B";
parameters["pName"] = "Contoso";
parameters["pType"] = 1; //object type code
//set required attendees
parameters["partyid"] = "2878282E-94D6-E111-9B1D-00155D9D700B";
parameters["partyname"] = "Contoso";
parameters["partytype"] = 1; //object type code
//open new record
entityFormOption.entityName = "appointment";
entityFormOption.entityId = null;
entityFormOption.openInNewWindow = true;
//open the entity record
Xrm.Navigation.openForm(entityFormOption, parameters, successCallback, errorCallback);
Thanks!