ApiPlatform - change identifier of an entity

I have an doctrine entity Tag defined in xml file like:

<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping https://www.doctrine-project.org/schemas/orm/doctrine-mapping.xsd">
    <entity name="App\Entity\Tag" table="`tag`">
        <id name="id" type="bigint" column="id">
            <generator strategy="SEQUENCE"/>
            <sequence-generator sequence-name="tag_id_seq" allocation-size="1" initial-value="1"/>
        </id>
        <field name="name" type="string" column="name" length="255" nullable="true">
            <options>
                <option name="fixed"/>
            </options>
        </field>
        <field name="slug" type="string" column="required" nullable="false">
        </field>
    </entity>
</doctrine-mapping>

I want to use slug as identifier not an id and I get an error when returning the collection:

No identifier defined in "App\Entity\Tag". You should add # [\ApiPlatform\Core\Annotation\ApiProperty(identifier: true)]" on the property identifying the resource.

Where it should be added? I have a problem of where it should be defined as I can not find any example with xml files

Also, found THIS.