To cite essurvey in publications use:

  Cimentada, Jorge (2019). Download Data from the European Social
  Survey on the Fly R package version 1.0.3

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Download Data from the European Social Survey on the Fly},
    author = {Jorge Cimentada},
    year = {2019},
    note = {R package version 1.0.3},
    url = {https://docs.ropensci.org/essurvey/},
  }