To cite expertsurv in publications use:

  Cooney P, White A (2023). _expertsurv: Incorporate Expert Opinion
  with Parametric Survival Models_. R package version 1.3.0.

A BibTeX entry for LaTeX users is

  @Manual{expertsurv-package,
    title = {expertsurv: Incorporate Expert Opinion with Parametric Survival Models},
    author = {Philip Cooney and Arthur White},
    year = {2023},
    note = {R package version 1.3.0},
  }

  Philp Cooney, Arthur White (2023). Direct Incorporation of Expert
  Opinion into Parametric Survival Models to Inform Survival
  Extrapolation Medical Decision Making, 1-12. URL
  https://journals.sagepub.com/doi/epub/10.1177/0272989X221150212

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Direct Incorporation of Expert Opinion into Parametric Survival Models to Inform Survival Extrapolation},
    author = {Philip Cooney and Arthur White},
    journal = {Medical Decision Making},
    year = {2023},
    pages = {1--12},
    url = {https://journals.sagepub.com/doi/epub/10.1177/0272989X221150212},
  }