The lister should be based on the assumption that the [[ https://scm.fusionforge.org/anonscm/gitweb?p=fusionforge/fusionforge.git;a=tree;f=src/plugins/repositoryapi;hb=refs/heads/feature/plugin-repositoryapi-6.0 | repository API plugin ]] is enabled on the target forge. (Which, in turn, requires a fusionforge version >= 6.)
The [[ https://scm.fusionforge.org/anonscm/gitweb?p=fusionforge/fusionforge.git;a=blob;f=src/plugins/repositoryapi/Documentation/API_documentation.md;hb=HEAD | API documentation ]] of the plugin is available from the same repo.
As an example, it is deployed on the Adullact forge: https://gforge.inria.fr/soap/?wsdl=1