It is not supported in the current GUI version. I can add it to the 2.1.0 beta version.
As a workaround you can initialize the repository and the storage using the CLI version and then the GUI version will be able to load it correctly once you restart Duplicacy.
On Windows, the Duplicacy CLI executable can be found under either C:\Program Files (x86)\Duplicacy
or C:\Users\username\AppData\Local\Duplicacy
, depending on whether you installed Duplicacy for all users or for the current user only.
On macOS, the Duplicacy CLI executable is usually under /Applications/Duplicacy.app/Contents/Resources
.