Access config.php (Nextcloud) to add trusted domain

[vetmedin]

Hi, from my question you can probably see, that I only got this far by a lot of googling, but well...

I want to add a trusted domain for Nextcloud in the config.php file. The problem is, I have no idea how to access this file. I found the default path and it seems to be somewhere I can´t just access using the Explorer (or can I?)
So can anyone tell me how I can get to this file to modify it?

Using latest TrueNAS Core.

Thanks!