The option is there, so I feel it should be possible...
If I start an on-premises web client, I can go to the Extension marketplace and select an app. I fill out my data, Then, when I want to start installing it, the system gives me the following message:
"Installation of extensions from the marketplace is not supported for your system configuration. For more information, see https://docs.microsoft.com/en-us/dynamics365/business-central/ui-extensions"
So, what is wrong with my system configuration? It's totally default installation (so, windows logon, cronus database, I installed our own license), and I seem unable to find what the prerequisites are to get this going.