First of all, you have to create a new module. After that, you have to put the code in config.xml
<adminhtml> <events> <adminhtml_block_salesrule_actions_prepareform> <observers> <magemonkeys_custofield> <type>model</type> <class>Magemonkeys_Custofield_Model_Observer</class> <method>adminhtmlBlockSalesruleActionsPrepareform</method> </magemonkeys_custofield> </observers> </adminhtml_block_salesrule_actions_prepareform> </events> </adminhtml>
in Model/Observer.php file you have to put this method and code.
<?php class Magemonkeys_Custofield_Model_Observer { public function adminhtmlBlockSalesruleActionsPrepareform($observer) { $fieldset = $observer->getForm()->getElement('action_fieldset'); $fieldset->addField('sku', 'text', array( 'name' => 'sku', 'label' => Mage::helper('freeproduct')->__('Add skus'), 'title' => Mage::helper('freeproduct')->__('Add skus'), 'note' => Mage::helper('freeproduct')->__('Add sku(s) separated with comma'), )); } } ?>
And after that, you have to flush the cache and check.