How to add a new field in cart price rule form in magento 2 backend?

Question

I need to add a new dropdown field in the "CART PRICE RULE" form in Backend:

MARKETING > PROMOTIONS > CART PRICE RULES > ADD NEW RULE

Answer 1

Create file in your custom module app/code/Vendor/Module/view/adminhtml/ui_component/sales_rule_form.xml

<?xml version="1.0" encoding="UTF-8"?>
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
    <fieldset name="rule_information" sortOrder="10">
        <field name="store_ids">
            <argument name="data" xsi:type="array">
                <item name="options" xsi:type="object">Magento\Cms\Ui\Component\Listing\Column\Cms\Options</item>
                <item name="config" xsi:type="array">
                    <item name="dataType" xsi:type="string">int</item>
                    <item name="label" xsi:type="string" translate="true">Store View Test</item>
                    <item name="formElement" xsi:type="string">multiselect</item>
                    <item name="source" xsi:type="string">sales_rule</item>
                    <item name="dataScope" xsi:type="string">store_ids</item>
                    <item name="default" xsi:type="string">0</item>
                    <item name="validation" xsi:type="array">
                        <item name="required-entry" xsi:type="boolean">true</item>
                    </item>
                </item>
            </argument>
        </field>
    </fieldset>
</form>

And you can see changes (the Store View Test column):

Change it to the desired one. But keep in mind, that this field just rendered in the layout, but to do save and load your custom data you should write a custom plugin code which can do this during the model load and save process.

Answer 2

To anyone who needs a complete answer to this question. first, create an installSchema or UpgradeSchema script in your custom module:

class UpgradeSchema implements UpgradeSchemaInterface
{

    /**
     * Upgrades DB schema for a module
     *
     * @param SchemaSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     */
    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
    {
       $installer = $setup;
       $installer->startSetup();
        if (version_compare($context->getVersion(), '1.0.3', '<')) {

            $installer->getConnection()->addColumn(
                $installer->getTable('salesrule'),
                'custom_field',
                [
                    'type' => \Magento\Framework\DB\Ddl\Table::TYPE_SMALLINT,
                    'length' => 1,
                    'unsigned' => true,
                    'nullable' => true,
                    'default' => '0',
                    'comment' => 'custom_field'
                ]
            );
        }
       $installer->endSetup();
    }

second, create the file

view/adminhtml/ui_component/sales_rule_form.xml

<?xml version="1.0" encoding="UTF-8"?>
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
    <fieldset name="rule_information" sortOrder="10">
        <field name="custom_field" formElement="select">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="source" xsi:type="string">custom_field</item>
                </item>
            </argument>
            <settings>
                <dataType>number</dataType>
                <label translate="true">custom field</label>
                <visible>true</visible>
                <dataScope>custom_field</dataScope>
            </settings>
            <formElements>
                <select>
                    <settings>
                        <options>
                            <option name="0" xsi:type="array">
                                <item name="value" xsi:type="number">1</item>
                                <item name="label" xsi:type="string" translate="true">Yes</item>
                            </option>
                            <option name="1" xsi:type="array">
                                <item name="value" xsi:type="number">0</item>
                                <item name="label" xsi:type="string" translate="true">No</item>
                            </option>
                        </options>
                    </settings>
                </select>
            </formElements>
        </field>
    </fieldset>
</form>

and that's it you're done