To cite PLmixed in publications use:

  Rockwood, N. J., & Jeon, M. (2019). Estimating complex measurement
  and growth models using the R package PLmixed. Multivariate
  Behavioral Research, 54(2), 288-306.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Estimating Complex Measurement and Growth Models Using the {R} Package {PLmixed}},
    author = {Nicholas Rockwood and Minjeong Jeon},
    journal = {Multivariate Behavioral Research},
    year = {2019},
    volume = {54},
    number = {2},
    pages = {288--306},
  }