To cite preference in publications use:

  Cameron B, Kane MJ, Esserman D (2020). “preference: An R Package for
  Two-Stage Clinical Trial Design Accounting for Patient Preference.”
  _Journal of Statistical Software, Code Snippets_, *94*(2), 1-16.
  doi:10.18637/jss.v094.c02 <https://doi.org/10.18637/jss.v094.c02>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{preference}: An {R} Package for Two-Stage Clinical Trial Design Accounting for Patient Preference},
    author = {Briana Cameron and Michael J. Kane and Denise Esserman},
    journal = {Journal of Statistical Software, Code Snippets},
    year = {2020},
    volume = {94},
    number = {2},
    pages = {1--16},
    doi = {10.18637/jss.v094.c02},
  }