To cite rddapp in publications use:

Jin Z, Liao W, Papst I, Zhang W, Hochstedler K, Thoemmes F (2021). rddapp: Regression Discontinuity Design Application. R package version 1.3.

Corresponding BibTeX entry:

    title = {rddapp: Regression Discontinuity Design Application},
    author = {Ze Jin and Wang Liao and Irena Papst and Wenyu Zhang and
      Kimberly Hochstedler and Felix Thoemmes},
    year = {2021},
    note = {R package version 1.3.},