Why do you want to get rid of that notification in the browser? This is Admin Console, so it usually only affects admins. Unless you are using some plugin like Ofmeet and users go to that URL. Of course, same certificate is used for clients connection. So a more sophisticated clients will show a warning. In Spark you will need to enable “Accept All certificates” to be able to connect.
Yes, you can use a third party cert, but i can’t help with that. Never used myself. I don’t know if it is possible to export a self-signed certificate either.
P.S. in future do not ask for prompt replies. I know you probably didn’t mean anything bad, but as we are all here just volunteers it sounds rude to demand quick reply