Below is my program :- 

@RestResource(URLMapping='/SendEmailForOTP/*')
global class WordpressIntegrationForEmail {
    
    @HTTPPOST
    global static FinalReturnWrapper SendEmailMsg(string email , string otp,string mob,boolean send_otp_if_exists)
    {
        
        FinalReturnWrapper returnResponse = new FinalReturnWrapper(); 
        cls_data objData = new cls_data();
        contact_data cobjData = new contact_data();
        string failed = 'Email not exist';   //Need to create custom label
        string success = 'Email successfully sent to email address' ;//Need to create custom label
        list<contact> conList = [Select id,Do_Not_Mail__c,Email_Verification_Status__c, Email_Verification_Status_Date__c from contact where email =: email Limit 1];
        
        if(conList.size() > 0 )  // If the email id is exist then ...
        {
            objData.contact_exists = true;
            
            for(contact c : conList)
            {
                objData.contact_id =c.id;
                cobjData.name=c.Name;
                if(c.Do_Not_Mail__c == false && c.Email_Verification_Status__c == null && c.Email_Verification_Status_Date__c == null)
                {
                    
                    OtpViaEmail.sendEmail(email,otp);
                    objData.otp_sms_sent = true; 
                    objData.otp_email_sent = true;
                    
                }
                
            }
            returnResponse.obj = objData;
            returnResponse.cobj= cobjData ;
            return returnResponse;
            
        }
        else    // If the email id not exist then...
        {
            if(send_otp_if_exists ==false )
            {
                OtpViaEmail.sendEmail(email,otp);
                objData.otp_sms_sent = true; 
                objData.otp_email_sent = true;
                
                
                return returnResponse;
            }
            
            
        }
        
    }
    
    global class FinalReturnWrapper {
        
        public cls_data obj ;
        public contact_data cobj;
        
    }
    public class cls_data {
        
        public boolean contact_exists;
        public boolean otp_sms_sent;
        public boolean otp_email_sent;
        public string contact_id;
        
    }
    
    public class contact_data {
        public string name;
        
        
    }
    
    
}


See here i need to return FinalReturnWrapper but when i called this from postman so getting error as parser error.

So can someone please let me know how can I change the response to json file. Please guid me the code.