When you're citing something from SSRN, first check to see if it has been published.
See Bluebook rules 17.3 (Forthcoming Publications), 17.4 (Working Papers), and 17.5 (Electronic Media and Online Sources).
When you are viewing an abstract, the navigation bar gives a URL like this:
But if you click on the link icon, you'll get a shorter URL, like this:
You can also find the shorter URL in the Suggested Citation:
Suggested Citation box for Robots in American Law, by Ryan Calo
Even though the Bluebook has an example with the long URL (R. 17.4), use the shorter one.
Prof. Calo's paper could be cited:
Ryan Calo, Robots in American Law (Univ. of Wash. Sch. of Law Research Paper No. 2016-04), https://ssrn.com/abstract=2737598.