...
If you need to install into an already existing UM you probably need to perform a manual installation as described in this section.
Go to https://www.universal-messenger.de/knowledge-base/intern/02_downloads/addons/index.html and download the Login App package (ZIP file).
...
Configure and install REST Proxy
Follow https://wwwdownloads.universal-messenger.de/knowledge-base/interndocumentation/doc-enuniversal-messenger/html/UM_EN/Administration/REST-proxy.html to install and configure your REST-Proxy.
...