You will need to contact Mathworks Sales for custom trials.
Mathworks will probably be reluctant to grant a trial for such an old version, as they no longer sell that version, only the most recent versions. The way it works for non-trials is that unless you have a Student license, if you have a license for a particular release, then you can install and use any previous release as well, using the same license. So you would not purchase an R2016a license now: you would purchase an R2020b license which would also permit you to use R2016a.
But that doesn't work for trial licenses; your sales person needs to provide you with the older software specifically.
