Contact Reference External Identifier
Small CiviCRM extension that adds the Contact External Identifier as an option to Autocomplete Contact Search and Contact Reference Options
Install
git clone https://github.com/mecachisenros/contactrefexternalidentifier.git in your site extension folder or download the zip file and upload it to you site extension folder and unzip
In CiviCRM, navigate to Administer - System settings - Extensions click the refresh button, you should see the extension, click Install/Enable.
Now you just need to go to Administer - Customize data and screens - Search preferences and check the External Identifier option