Mongoct, The Hansgrohe iBox rough-in has a large circular rubber like facing that gets affixed to the cement board encircling the rough-in. Then another coat is to be applied on top of the facing. I called the tech department at Hansgrohe to find out what to use, and guess what, they don't know :/

What do I use that will allow a coating of Hydroban on top of it, once I'm done with the fixative layer?

I have linked a web page from hansgrohe that has a downloadable brochure (6.2mb) that states to use an "adhesive" but that seems broad to me. See page 8. The "glossy" brochure is a largish file. I am assuming that an adhesive with silicone would be a problem with the Hydroban covering on top. But I really don't know. There is also a video on the ibox on this same web page.

Here is a link that might be useful: Hansgrohe web site iBox