MyDataObject.php

<?php

class MyDataObject extends DataObject {
	
	public function getCMSFields() {
		$fields = parent::getCMSFIelds();
		
		$clientSource = function() {
			return Client::get()->map('ID', 'ClientName');
		};
		
		$clientIDField = new DropdownField('ClientID', 'Client', $clientSource());
		$clientIDField->useAddNew('Client', $clientSource);
		
		$fields->addFieldToTab('Root.Foo', $clientIDField);
		return $fields;
	}
	
}