Fish is a shell

alias NAME=DEFINITION
alias kubectl_test="kubectl --kubeconfig=/Users/katrin/.kube/config_test"

If you want to persist this alias, use the --save flag:

alias --save kubectl_test="kubectl --kubeconfig=/Users/katrin/.kube/config_test"

This will also save your alias-function in the fish configuration directory in (unless specified differently) ~/.config/fish/functions

Now you can enjoy your new alias like me: kubectl_test get namespace

https://fishshell.com/docs/current/cmds/alias.html#:~:text=fish%20marks%20functions%20that%20have,be%20erased%20using%20functions%20%2De%20.