// If you're using regular BibTex
@misc{RandomOrg,
  author = {Mads Haahr},
  title = {{RANDOM.ORG:} True Random Number Service},
  howpublished = {\url{https://www.random.org}},
  year = 1998--2018,
  note = {Accessed: 2018-06-01}
}
// If you're using BibLaTeX
@online{RandomOrg,
  author = {Mads Haahr},
  title = {{RANDOM.ORG:} True Random Number Service},
  year = 1998--2018,
  url = {https://www.random.org},
  urldate = {2018-06-01}
}