I was having this problem too. In HKCU\SOFTWARE\Microsoft\Edge\NativeMessagingHosts I found "com.webex.meeting".
In GPO, add that to the setting Computer Configuration\Policies\Administrative Templates\Microsoft Edge\Native Messaging set "Control which native messaging hosts users can use" to "com.webex.meeting".
Then set "Configure native messaging block list to "*". Leave the "allow user-level messaging hosts..." not configured. That worked for me.