To cite assignR in publications use:

  Chao Ma, Hannah B. Vander Zanden, Michael B. Wunder, and Gabriel J.
  Bowen (2020). assignR: An R package for isotope-based geographic
  assignment. Methods in Ecology and Evolution.
  https://doi.org/10.1111/2041-210X.13426

A BibTeX entry for LaTeX users is

  @Article{,
    title = {assignR: An R package for isotope-based geographic assignment},
    author = {{Ma Chao} and {Vander Zanden Hannah B.} and {Wunder Michael B.} and {Bowen Gabriel J.}},
    journal = {Methods in Ecology and Evolution},
    year = {2020},
    doi = {10.1111/2041-210X.13426},
    url = {https://doi.org/10.1111/2041-210X.13426},
  }