When I right click on file name I get a menu with amongst others "Share". When I hover over this option a help text appears saying "Choose an app to share the selected file". When I click the share option nothing happens.
How do I define an app to be used when I want to share a file?
Use the Send To menu instead of the Share menu.
(The Share menu has never worked outside of Explorer and should be blocked from appearing, but apparently keeps being changed such that the block doesn't work.)
If you type /sendto into the path field and push return, you'll be taken to a folder where you can create shortcuts to programs to add them to the menu. This works throughout Windows, not just in Opus.
You can also add context menu items for specific apps via Settings > File Types, if you want them at the top level of the menu.
Or you can create toolbar buttons or hotkeys which do the same thing. (Settings > Customize Toolbars, then drag the program to a toolbar.)
Thanks for your input. I wanted to use this to send a file (picture) to one of my Whatsapp contacts. I did what you said and copied the Whatsapp shortcut to the /sendto directory. When I right click a picture and choose send to I can click Whatsapp. Whatsapp starts, but that's it. When I select a contact in Whatsapp the picture cannot be pasted.
I wanted to use share as it works in many other applications. Too bad I cannot do this from Directory Opus. Thanks anyway for your suggestion.
FWIW, for WhatsApp you can drag & drop files on to it to share them with people, at least for the web version. I think the Windows app version is the same thing, just inside its own window.