To cite survextrap in publications use:

Christopher Jackson survextrap: a package for flexible and transparent survival extrapolation BMC Medical Research Methodology (2023)23:282

Corresponding BibTeX entry:

  @Article{,
    title = {survextrap: a package for flexible and transparent
      survival extrapolation},
    author = {Christopher Jackson},
    journal = {BMC Medical Research Methodology},
    year = {2023},
    volume = {23},
    number = {282},
    doi = {10.1186/s12874-023-02094-1},
  }