Hello, how to set use splitter permission by default for all players please? 

Grant it to the default group. We have a permissions tutorial under the Docs I’d suggest taking a look at.
https://umod.org/documentation/plugins/permissions

o.grant group default furnacesplitter.use

But do yourself a favor and read, read and read some more. Will really save you time in the long run.

Thanks boys, sorry I'm new and I didn't saw the documentation!