synapse-app-mas/POSTINSTALL.md

Account ids are created with the username and the second level domain under which the app is installed e.g. @$CLOUDRON-USERNAME@$CLOUDRON-APP-DOMAIN.

For federation to work, the second level domain ($CLOUDRON-APP-DOMAIN) has to be configured to serve up the .well-known/matrix/server URI. See the federation docs on how to do this.