How to add Custom Field on Particular Object through Salesforce Apex Class?

Please try this it will work.

public static void createObject()
{
MetadataService.MetadataPort service = createService();
MetadataService.CustomObject customObject = new MetadataService.CustomObject();
customObject.fullName = 'Test__c';
customObject.label = 'Test';
customObject.pluralLabel = 'Tests';
customObject.nameField = new MetadataService.CustomField();
customObject.nameField.type_x = 'Text';
customObject.nameField.label = 'Test Record';
customObject.deploymentStatus = 'Deployed';
customObject.sharingModel = 'ReadWrite';
List<MetadataService.SaveResult> results =
service.createMetadata(
new MetadataService.Metadata[] { customObject });
handleSaveResults(results[0]);
}

public static void createField()
{
MetadataService.MetadataPort service = createService();
MetadataService.CustomField customField = new MetadataService.CustomField();
customField.fullName = 'Test__c.TestField__c';
customField.label = 'Test Field';
customField.type_x = 'Text';
customField.length = 42;
List<MetadataService.SaveResult> results =
service.createMetadata(
new MetadataService.Metadata[] { customField });
handleSaveResults(results[0]);
}