When prompted, specify? Expand = attributes.value, then the value field of the additional field will return completely (not just the meta and name).
https://online.moysklad.ru/api/remap/1.1/entity/product/ade4468e-4409-11e9-ac12-000d000000c0?expand=attributes.value
The answer will be
"attributes": [ { "meta": { "href": "https://online.moysklad.ru/api/remap/1.1/entity/product/metadata/attributes/8b0bfb6c-459e-11e9-ac12-000d0000001f", "type": "attributemetadata", "mediaType": "application/json" }, "id": "8b0bfb6c-459e-11e9-ac12-000d0000001f", "name": "Π‘ΠΏΡΠ°Π²ΠΎΡΠ½ΠΈΠΊ", "type": "customentity", "value": { "meta": { "href": "https://online.moysklad.ru/api/remap/1.1/entity/customentity/906e24fb-459d-11e9-ac12-000d00000018/795f38c1-459e-11e9-ac12-000d0000001c", "metadataHref": "https://online.moysklad.ru/api/remap/1.1/entity/companysettings/metadata/customEntities/906e24fb-459d-11e9-ac12-000d00000018", "type": "customentity", "mediaType": "application/json", "uuidHref": "https://online.moysklad.ru/app/#custom_906e24fb-459d-11e9-ac12-000d00000018/edit?id=795f38c1-459e-11e9-ac12-000d0000001c" }, "id": "795f38c1-459e-11e9-ac12-000d0000001c", "accountId": "56c92164-40d9-11e9-ac12-000a00000001", "updated": "2019-03-13 17:44:15", "name": "ΠΠ΄ΠΈΠ½ΠΈΡΠ°", "description": "ΠΠΏΠΈΡΠ°Π½ΠΈΠ΅ Π΅Π΄ΠΈΠ½ΠΈΡΡ", "externalCode": "O86DR1gHha9dNwBS-jW253" } } ]