Serviio

I am looking for a possibility to set a password for the Serviio server (port 23423)?

I can only set a password for the mediabrowser (23424).

Can anyone help me?

In a nutshell, no, not from within Serviio. You could do it with other tools eg. reverse proxies.
Search the forum - its been discussed a bit - eg: viewtopic.php?f=5&t=21440