I am not sure if you can install plugins in your system. If you are using a shared plan then probably not. If you have a VPS or dedicated server, then probably yes. But, you need a plugin for roundcube to do that. It does not do it automatically.
If it is a shared plan then you need to request customer service to install it. I do not know if they will or not as it is up to them. But, you probably can't install a plugin on a shared plan.
The plugin is at the web site below.
Look for "automatic addressbook plugin".
Of course, the alternative is to first add the contact to the address book and then send the mail from the contacts list....