Hey,
Just an update, I successfully got this working by using the overrides capability of the .Net Client.
For anyone else trying to do the same thing, here is an example:
var renderRequest = new RenderRequest { Template = ... // your template Data = ... // your data Overwrites = new { Template = new { PdfPassword = new { OwnerPassword = "some password", Printing = "highResolution" // you can also add the other fields specified at // https://jsreport.net/learn/pdf-utils#password-protection } } } }