hroot is a citeware, that means you can use the software free of change if you sign the hroot license agreement and cite it this way:
The experiment was organized und administrated with the software hroot (Bock et al., 2014).
Bock, Olaf, Ingmar Baetge & Andreas Nicklisch (2014). hroot – Hamburg registration and organization online tool. European Economic Review 71, 117-120
hroot licence agreement