To cite metapower in publications use:

  Griffin, J. W. (2021). Calculating statistical power for
  meta-analysis using metapower. The Quantitative Methods for
  Psychology, 17(1), 24–39. doi:10.20982/tqmp.17.1.p024

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Calculating statistical power for meta-analysis using metapower},
    author = {Jason W. Griffin},
    journal = {The Quantitative Methods for Psychology},
    volume = {17},
    number = {1},
    pages = {24-39},
    year = {2021},
    doi = {10.20982/tqmp.17.1.p024},
    url = {https://CRAN.R-project.org/package=metapower},
  }