In the backend config, there is only support for a container name.
The azure blob storage does not seem to directly support folders (the equivalent would be containers). But, you can add slashes in blob names (equivalent of filenames) to add a virtual hierarchy (this is from a brief look at the documentation, I am not familiar with it myself).
What you can try, is using the advanced option --prefix=folder/duplicati. Make sure that every prefix is different. Do a test backup and restore, and if you can, look at the container in azure to see if everything is as expected.
If the slash does not work, use different prefixes without folders. However, depending on the internal implementation, it might be required to list the contents of the entire container for every backup. If that is the case, you might see slowdowns if many backups share the same container.