The sample code below returns the default entity id of an account and sets the autoSubmit to true so that users cannot change the default billing entity on quotes.

1. Add a custom field, DefaultEntity__c, on the Account object.

2. Populate the DefaultEntity__c field with an entity Id.

3. Use the following code to implement the custom SelectEntity plugin.

4. Register the plugin . See step 2 under the Procedure section.

5. Create a new quote and see the entity field pre-populated with the default entity you gave in Step #2.

global without sharing class MySelectEntityPlugin implements 
   zqu.SelectEntityComponentOptions.ISelectEntityPlugin {
      global zqu.SelectEntityComponentOptions.SelectEntityPluginResponse 
    ​     selectEntityForNewQuote(String accountId, String opportunityId) {
            zqu.SelectEntityComponentOptions.SelectEntityPluginResponse resp = 
            ​   new zqu.SelectEntityComponentOptions.SelectEntityPluginResponse();
            Account acc = 
            ​   [SELECT Id,DefaultEntity__c FROM Account WHERE Id=:accountId LIMIT 1];
            if(acc.DefaultEntity__c != null) {        
               resp.selectedId = acc.DefaultEntity__c;
               resp.autoSubmit = true;
            }
            return resp;
      }
}