BibTeX:
@incollection{Bahullh2002,
  author = {Bahá'u'lláh},
  title = {The King in the Box},
  booktitle = {The Summons of the Lord of Hosts},
  year = {2002},
  publisher = {Bahá'í World Centre},
  url = {https://www.bahai.org/library/authoritative-texts/bahaullah/summons-lord-hosts/},
}

APA:
Bahá'u'lláh. (2002). The King in the Box. In *The Summons of the Lord of Hosts*. Bahá'í World Centre.

MLA:
Bahá'u'lláh. "The King in the Box." *The Summons of the Lord of Hosts*, Bahá'í World Centre, 2002.

Chicago:
Bahá'u'lláh. "The King in the Box." In *The Summons of the Lord of Hosts*, Bahá'í World Centre, 2002.