At the moment it is not possible to configure roundcube as default mail client for LibreOffice.
LibreOffice on Linux can be configured in Tools -> Options -> Internet -> Email.
LibreOffice on Windows is taking default MAPI application.
In both cases only installed applications are visible, no webmail client.
Firefox Webbrowser can be configured with add-ons like "mailtowebmail" (and Roundcube Template) to use a webmail client for all internal mailto links.
Our expectation is:
It should be possible to use roundcube webmail on Linux and Windows Clients as default mail client for LibreOffice and Firefox.