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In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity_name" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

Here item name custom_import_entity_name should be similar to the entity name defined in Namespace/ModuleName/etc/export.xml file as in below example.

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_ImportExport:etc/export.xsd">
    <entity name="custom_import_entity_name" label="Products" model="Magento\CatalogImportExport\Model\Export\Product" entityAttributeFilterType="catalog_product"/>
    <entityType entity="custom_import_entity_name" name="simple" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <entityType entity="custom_import_entity_name" name="virtual" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <fileFormat name="csv" label="CSV" model="Magento\ImportExport\Model\Export\Adapter\Csv" />
</config>

Sample CSV file path: Namespace/ModuleName/Files/Sample/custom_import_entity_name.csv

In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

Here item name custom_import_entity should be similar to the entity name defined in Namespace/ModuleName/etc/export.xml file as in the below example.

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_ImportExport:etc/export.xsd">
    <entity name="custom_import_entity" label="Products" model="Magento\CatalogImportExport\Model\Export\Product" entityAttributeFilterType="catalog_product"/>
    <entityType entity="custom_import_entity" name="simple" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <entityType entity="custom_import_entity" name="virtual" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <fileFormat name="csv" label="CSV" model="Magento\ImportExport\Model\Export\Adapter\Csv" />
</config>

Also, make sure to add the below method in your export entity model class specified in the export.xml file above.

/**
     * EAV entity type code getter.
     *
     * @return string
     */
    public function getEntityTypeCode()
    {
        return 'custom_import_entity';
    }

Sample CSV file path: Namespace/ModuleName/Files/Sample/custom_import_entity.csv

In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity_name" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

Sample file path  : Namespace/ModuleName/Files/Sample/custom_import_entity_name.csv

In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity_name" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

Here item name custom_import_entity_name should be similar to the entity name defined in Namespace/ModuleName/etc/export.xml file as in below example.

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_ImportExport:etc/export.xsd">
    <entity name="custom_import_entity_name" label="Products" model="Magento\CatalogImportExport\Model\Export\Product" entityAttributeFilterType="catalog_product"/>
    <entityType entity="custom_import_entity_name" name="simple" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <entityType entity="custom_import_entity_name" name="virtual" model="Magento\CatalogImportExport\Model\Export\Product\Type\Simple" />
    <fileFormat name="csv" label="CSV" model="Magento\ImportExport\Model\Export\Adapter\Csv" />
</config>

Sample CSV file path: Namespace/ModuleName/Files/Sample/custom_import_entity_name.csv

In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity_name" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

In Magento 2.3 onwards, the sample file can be specified from custom module's di.xml file as below:

<type name="Magento\ImportExport\Model\Import\SampleFileProvider">
        <arguments>
            <argument name="samples" xsi:type="array">
                <item name="custom_import_entity_name" xsi:type="string">Namespace_ModuleName</item>
            </argument>
        </arguments>
    </type>

Sample file path : Namespace/ModuleName/Files/Sample/custom_import_entity_name.csv