I've had the same issue trying to have a specific profile for Photo with the Huion tablet driver. I couldn't find the executable and trying to follow the recommendation on this post about selecting it from the %AppData% doesn't work. Fortunately Huion had already an answer. It appears Affinity have moved to a UWP, that is a universal Windows platform (yes, technically you can run Affinity on an X-Box or ARM based machine). So instead what you'll need is add it from the "Windows Programs" which is found in the C:\Program Files\. The folder is hidden and when you make it show in explorer you might not be able to access it, you'll need to add yourself as an owner to the folder and from there on you can select the cryptic folder name and eventually the actual executable file for photos, designer or publisher.
Here is the original link (Note, those of you on Win 11 will notice a slight difference to the workflow, don't bother with it, the instructions remain the same): https://support.huion.com/en/support/solutions/articles/44002288046-how-to-set-different-shortcuts-keys-for-uwp-software-on-huion-driver
Cheers