Add custom data to SalesRule API

Question

I tried to add custom data to SalesRule through the endpoint V1/salesRules but ended up with getting error

My code is below

1- etc/extension_attributes.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\SalesRule\Api\Data\RuleInterface">
        <attribute code="customer_email" type="string"/>
        <attribute code="domain_name" type="string"/>
        <attribute code="phone_number" type="string"/>
    </extension_attributes>
</config>

2- di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
&lt;type name=&quot;Magento\SalesRule\Api\RuleRepositoryInterface&quot;&gt;
    &lt;plugin name=&quot;vendor_extend_cart_rule_add_custom_data_to_sales_rule&quot; type=&quot;Vendor\ExtendedCartRule\Plugin\AddCustomDataToSalesRule&quot;/&gt;
&lt;/type&gt;


</config>

3- Plugin

<?php
declare(strict_types=1);
namespace Vendor\ExtendedCartRule\Plugin;
use Magento\SalesRule\Model\ResourceModel\Rule\Collection;
use Magento\SalesRule\Model\ResourceModel\Rule\CollectionFactory;
use Magento\SalesRule\Api\RuleRepositoryInterface;
class AddCustomDataToSalesRule
{
    private $techwareSalesRuleCollectionFactory;
public function __construct(
    CollectionFactory $techwareSalesRuleCollectionFactory
) {
    $this-&gt;techwareSalesRuleCollectionFactory = $techwareSalesRuleCollectionFactory;
}

/**
 * @param SalesRuleRepositoryInterface $subject
 * @param $result
 * @return mixed
 */
public function afterGet(
    RuleRepositoryInterface $subject,
    $result
) {


    $SalesRuleCollection = $this-&gt;techwareSalesRuleCollectionFactory-&gt;create();
    $techwareSalesRule = $SalesRuleCollection
        -&gt;addFieldToFilter('rule_id', $result-&gt;getId())
        -&gt;getFirstItem();

    $extensionAttributes = $result-&gt;getExtensionAttributes();

    $extensionAttributes-&gt;setData('customer_email', $techwareSalesRule-&gt;getData('customer_email'));
    $extensionAttributes-&gt;setData('domain_name', $techwareSalesRule-&gt;getData('domain_name'));
    $extensionAttributes-&gt;setData('phone_number', $techwareSalesRule-&gt;getData('phone_number'));

    $result-&gt;setExtensionAttributes($extensionAttributes);

    return $result;
}

/**
 * @param SalesRuleRepositoryInterface $subject
 * @param $result
 * @return mixed
 */
public function afterGetList(
    RuleRepositoryInterface $subject,
    $result
) {

    foreach ($result-&gt;getItems() as $sales_rule) {
        $this-&gt;afterGet($subject, $sales_rule);
    }

    return $result;
}

}

Here is the structure of Salesrule table after altering and adding custom column.