Linux doesn’t work like Windows. You need to mount your drive for it to be usable by the system, it can be done temporarily by trying to access it from the file manager usually, but this is temporary and after reboot it will not be automatically mounted.
So first you need to make sure your drive is automatically mounted on boot, so it has a permanent access path in the system. When it is done, you can use it in Steam.
Also make sure you have formatted it as an EXT4 partition so it works properly in Linux in general, and with Steam particularly (so it can have proper permission management and things like that).
//EDIT: depending on your desktop environment, you can also have a possibility to help you mount your drive more easily.