for(String username :listmaker.keySet()){

            if(username != null){
            sendgridsettings__c sendSetting = sendgridsettings__c.getInstance(username);
            system.debug(sendSetting);

            // Storing parameters to a call as separate parameters can make things a
            //   bit easier to read...but do incur a small amount of overhead.
            // I chose to serialize your payload data inline (as opposed to storing in
            //   a separate variable), but it really doesn't make any difference.
            // Use whatever you're more comfortable with (again, _be consistent_)
            SendGridAPI.sendRequest(json.serialize(listmaker.get(username)),
                                    '/v3/contactdb/lists/' + sendSetting.list_id__c + '/recipients',
                                    'POST');
            }
            else{
                string usernames = 'Roger';
                  sendgridsettings__c sendSetting = sendgridsettings__c.getInstance(usernames);
            system.debug(sendSetting);

            // Storing parameters to a call as separate parameters can make things a
            //   bit easier to read...but do incur a small amount of overhead.
            // I chose to serialize your payload data inline (as opposed to storing in
            //   a separate variable), but it really doesn't make any difference.
            // Use whatever you're more comfortable with (again, _be consistent_)
            SendGridAPI.sendRequest(json.serialize(listmaker.get(usernames)),
                                    '/v3/contactdb/lists/' + sendSetting.list_id__c + '/recipients',
                                    'POST');


            }