BibTeX:
@incollection{Various,
  author = {Various},
  title = {Shoghi Effendi's Letter to the German Friends},
  booktitle = {Baha'i Stories Blog},
  url = {https://bahaistories.blogspot.com/},
}

APA:
Various. Shoghi Effendi's Letter to the German Friends. In *Baha'i Stories Blog*.

MLA:
Various. "Shoghi Effendi's Letter to the German Friends." *Baha'i Stories Blog*, n.d..

Chicago:
Various. "Shoghi Effendi's Letter to the German Friends." In *Baha'i Stories Blog*, n.d..