BibTeX:
@incollection{Various,
  author = {Various},
  title = {Badí, the Messenger of Bahá'u'lláh},
  booktitle = {Baha'i Stories for Children},
  url = {https://bahaistoriesforchildren.blogspot.com/2019/08/badi-messenger-of-bahaullah.html},
}

APA:
Various. Badí, the Messenger of Bahá'u'lláh. In *Baha'i Stories for Children*.

MLA:
Various. "Badí, the Messenger of Bahá'u'lláh." *Baha'i Stories for Children*, n.d..

Chicago:
Various. "Badí, the Messenger of Bahá'u'lláh." In *Baha'i Stories for Children*, n.d..