Dynamic JSON String Format in Apex

@isTest static void test_getDifferentContact(){
Contact contactRecord = [SELECT Id, Email,LastName
FROM Contact
WHERE LastName = 'Test Contact'
LIMIT 1];

<strong>String jsonInput = '[{"ConvertedDate":null,"scheduledAt":"2020-11-09","id":'+' "' + contactRecord.Id +'" ' +' ,"email": ' +' "'+ contactRecord.Email + '" '+ ',"name":' + ' "' + contactRecord.LastName + '" ' + '}]';
system.debug('jsonInput'+jsonInput);
</strong>

Test.startTest();
sendGift.getDifferent('Contact', jsonInput,'createdbyid','{createdbyid} sent you a gift!','full-name');
Test.stopTest();
}

String jsonInput = '[{"ConvertedDate":null,"scheduledAt":"2020-11-09","id":'+' "' + contactRecord.Id +'" ' +' ,"email": ' +' "'+ contactRecord.Email + '" '+ ',"name":' + ' "' + contactRecord.LastName + '" ' + '}]';