Add Dopus as default "Photo viewer"

I have been fighting to have Dopus to be the default external photo viewer.
I have succeeded to have D8Viewer the default viewer via Windows File Explorer for some file types, but cannot add this viewer in the Windows 10 "Default App" as "Photo Viewer".
As it is now, it is important I leave this field empty!

Recommended method is here: How to use the Opus image viewer inside or outside of Opus

The problem with the more general "default photo viewer" setting in Windows is there are lots of types of photos/images, which not all viewers show, and the Opus viewer can also handle lots of file types which you may not all want to be sent to it. We feel it is better to set things up for each type/extension instead of all at once. (You're unlikely to want Word documents or MP3 files opening in our viewer on double-click, for example, even though it can handle them.)

When I set "Directory Dopus" as the Default App for "Photo viewer", opening an image from e.g. the mail program, the Dopus file explorer is opened.
Instead of showing Dopus explorer, I would like to select the Dopus viewer instead, but cannot figure out how to do this.

Opus itself (DOpus.exe) would not normally be offered as a default in that list. The Opus viewer may be (D8Viewer.exe).

But the Default Apps list in Windows is not what the guide I linked says to use, and not what we recommend.

In my Windows 10 Setup, this is not the situation.
Selecting the Directory Opus will open Opus File Explorer for the images views - not the Opus Viewer itself!
See attachment.

That could happen if you used Open With to assign dopus.exe to images, but that won't work, and is not what the guide says to do either.

Do what the guide says, and it will work.

Is it correct that I cannot use Directory Opus in this setting, even it shows up!
If I leave this item blank, I can assign a jpg, pgn, 'and so on ...' to open the D8viewer via the File Explorer right click option.
Still this option will will not show up in the "Default apps"
Is the individual assign of an extension the only solution to assign an image to open directly in the Opus Viewer?

Please just do what the guide says to do. I linked it above. It explains what to do to achieve what you want.