@Daniel Juravski there is no global setting that changes the default state of this checkbox. The state of the checkbox is saved per user/repository. For example, if you choose to delete branch after merge in repository ABC, on next merge the checkbox will be checked.
If you still need to have this checkbox checked by default, here are a few options:
If you could tell us more about your use-case that would be great - BSERV-9254
I would appreciate having it checked by default because I often forget to check it. Our teams tend to forget it as well and this has lead to use having a lot of dead branches that we have to clean up. They're not that hard to clean up, but they really pile up. This would be much easier if the box was checked by default. Thank you!