External ID: define/collect external ID

// current initialization 
// MeiroEvents.init(domain); 
// initialization with external_id 
MeiroEvents.init({ domain: domain, external_id: "some_external_id_1234" });

You can also define multiple external ids via JSON object passed to init function following way:

// current initialization 
// MeiroEvents.init(domain); 
// initialization with external_id 
MeiroEvents.init({ 
  domain: domain, 
  external_id: { 
    ga: "ga client id",
    app_cid: "site client id"
  }
});