A publication on fsbrain has been submitted, but is not published yet. For now, please cite the package as follows:

To cite package 'fsbrain' in a publications use:

Tim Schäfer (2020). fsbrain: Managing and Visualizing Brain Surface Data. R package version
0.3.0. https://CRAN.R-project.org/package=fsbra...
A BibTeX entry for LaTeX users is
@Manual{,
title = {fsbrain: Managing and Visualizing Brain Surface Data},
author = {Tim Schäfer},
year = {2020},
note = {R package version 0.3.0},
url = {https://CRAN.R-project.org/package=fsbrain},
doi = {10.5281/zenodo.3559816},
}

Adapt to the version you are acutally using.