Is the Narayanopanisha starting with “atha puruSO ha vai nArAyaNO” considered a prashnam on its own or is it part of a part of a prashnam of the Vedas?

Narayana Upanishad is separate on its own. Within this “atha puruSO ha vai nArAyaNO” is an anuvAkam. Narayana Upanishad is short, and it does not contain prashnams like the other Upanishad-s. There are only 4 or 5 cantos in it. And one of the cantos starts with “atha puruSO ha vai nArAyaNO”.