How can I install Boot Camp drivers in Windows on an external SSD disk?
Open Bootcamp Assistant (In the Applications -> Utilities folder), then on the opening screen, click the 'Action' menu, then select 'Download Windows Support Software'.  You'll be prompted to select a location to save the downloaded files.  You can then put them onto a USB flash drive, restart into Windows and run the installer.

There are more details on the Apple Support website, here