We can use matrix client to access to #rpmfusion on Libera.Chat
First to simply the process register you nickname on Libera.Chat
After login https://chat.fedoraproject.org add room @NickServ:libera.chat at @NickServ:libera.chat write IDENTIFY <your_user_libera.chat> <your_password_libera.chat>
nickserver replies with "You are now identified for username at libera"
and finally join to #libera_#rpmfusion-admin:matrix.org
register user on libera.chat via matrix.org it is possible but confuse see here https://fedoramagazine.org/access-freenode-using-matrix-clients/