BibTeX:
@incollection{AbdulBah,
  author = {'Abdu'l-Bahá},
  title = {A Letter to the Peacemakers: 'Abdu'l-Bahá's Tablet to The Hague},
  booktitle = {Selections from the Writings of 'Abdu'l-Bahá},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/ebooks/19287},
}

APA:
'Abdu'l-Bahá. A Letter to the Peacemakers: 'Abdu'l-Bahá's Tablet to The Hague. In *Selections from the Writings of 'Abdu'l-Bahá*. Bahá'í World Centre.

MLA:
'Abdu'l-Bahá. "A Letter to the Peacemakers: 'Abdu'l-Bahá's Tablet to The Hague." *Selections from the Writings of 'Abdu'l-Bahá*, Bahá'í World Centre, n.d..

Chicago:
'Abdu'l-Bahá. "A Letter to the Peacemakers: 'Abdu'l-Bahá's Tablet to The Hague." In *Selections from the Writings of 'Abdu'l-Bahá*, Bahá'í World Centre, n.d..