To cite optimr in publications use:

Nash JC (????). optimr: A Replacement and Extension of the 'optim' Function. https://CRAN.R-project.org/package=optimr.

Corresponding BibTeX entry:

  @Manual{,
    title = {{optimr}: A Replacement and Extension of the 'optim'
      Function},
    author = {John C Nash},
    url = {https://CRAN.R-project.org/package=optimr},
  }