I ask again, What Version of Silkypix are you using?
The Free Version? or Silkypix Developer Studio Pro or Developer Studio?
There are several versions of this software. Don't expect much from a free version of anything that is why it's free.
The included versions of the Silkypix software with Fuji and Pentax are very basic packages, and it would not surprise me if you can't save to prophoto.
In the Full version of the software, you can save to Prophoto,
per my screen shots, and you can give the file a custom name
. These screen shots are from the Silkypix Developer Studio software, not the "free" code.
Here is link to the site where the full code is located. https://silkypix.isl.co.jp/en/