How to Enable Additional Driver in elementary Juno 5.0?
Driver should be installed via appcentre. All the necessary required drivers are provided via appcenter.
If by additional you meant alternate driver. Then you could get it from website of manufacturer of hardware component. For example you could get an alternate (upstream) driver for Nvidia Graphics from Nvidia driver support website.
Otherwise if by additional you meant extra driver, then you might not need it, if its not being shown in appcenter.
If your are facing any issue with a particular hardware then you should directly search for its driver alternate.