Custom-fields-write-functions.html
<mvt:item name="customfields" param="Write_Product_Code( 'product_code', 'custom_field_code', 'value' )" />
<mvt:item name="customfields" param="Write_Product_ID( 1, 'custom_field_code', 'value' )" />
<mvt:item name="customfields" param="Write_Product_Code( 'product_code', 'custom_field_code', 'value' )" />
<mvt:item name="customfields" param="Write_Product_ID( 1, 'custom_field_code', 'value' )" />