Flatpak comes pre-installed in openSUSE Leap 15.2. However, executing the flatpak command for the first time had a minor quirk in a freshly installed Leap 15.2 machine.

error: While opening repository /var/lib/flatpak/repo: opendir(objects): No such file or directory

I see other users commenting about this on Reddit. So, for now, just remove the repo file.

sudo rmdir /var/lib/flatpak/repo

Then, add the Flathub repository as its documentation describes.

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

Voilà! Now, you may search apps and install them with the flatpak command. For example, if you would search for Visual Studio Code, you would do:

flatpak search code

Find its Application Id from the result obtained with the above command and install it as follows:

flatpak install flathub com.visualstudio.code

The installation command arguments format is:

flatpak install repo_name application_id

In order to remove an application that you previously installed through Flatpak, just replace install by remove followed by the Application Id and it's done.