Setting a default 'From' address when sending an email from Cases

global class EmailPublisherLoader implements QuickAction.QuickActionDefaultsHandler {  
   // Empty constructor  
   global EmailPublisherLoader() {  
   }  
   public string customLabelValue{get;set;}  
   // The main interface method  
   global void onInitDefaults(QuickAction.QuickActionDefaults[] defaults) {  
     QuickAction.SendEmailQuickActionDefaults sendEmailDefaults = null;  
     // Check if the quick action is the standard Case Feed send email action  
     for (Integer j = 0; j < defaults.size(); j++) {  
       if (defaults.get(j) instanceof QuickAction.SendEmailQuickActionDefaults &&   
         defaults.get(j).getTargetSObject().getSObjectType() ==   
           EmailMessage.sObjectType &&   
         defaults.get(j).getActionName().equals('Case.Email') &&   
         defaults.get(j).getActionType().equals('Email')) {  
           sendEmailDefaults =   
             (QuickAction.SendEmailQuickActionDefaults)defaults.get(j);  
           break;  
       }  
     }  
     if (sendEmailDefaults != null) {  
       Case c = [SELECT CaseNumber FROM Case   
            WHERE Id=:sendEmailDefaults.getContextId()];  
       EmailMessage emailMessage = (EmailMessage)sendEmailDefaults.getTargetSObject();    
       // Set From address to make sure each email goes from sales@inmobi.com  
       emailMessage.FromAddress = getfromAddress(c.CaseNumber);  
   }  
  }  
 // return email address, Email Address is a from Address and this value is stored in custom label Email Address Label  
 private String getfromAddress(String reason) {  
    customLabelValue = System.Label.Email_Address_Label ;  
                 return customLabelValue;   
     }    
 }  

After creating Apex class , Go to case support settings  -> choose the Class Name -> Save