In IBM Connections Desktop plugin, when we are sharing file externally and trying to add external email address then there is only option to search that email address in directory. However since this is an external email address, it can't be searched in directory and hence it can't be added. There should be an option to add the email address directly without searching in directory. Otherwise files can't be shared externally from Desktop plugin.
In browser, a "+" sign can be clicked to add external email and share file to external user.