if you mean the next cloud site, then you should use the admin user and password, that you entered when you installed next cloud.
Thats what I am using and it does nothing.
I tried thousands of configurations (Pebble’s too) and no luck here. I have to say that would like to access through truenas_certificate in order to get more security.
This trouble is driving me crazy and desperate.
Anyone with the solution?
Did you finally get it @pebble ?
Unfortunately no luck here aswell 
I tried @Benders solution and it worked as described (thank you for that) but as you said it is without https. If I “Edit” the working App installation from the truenas GUI and enable the default truenas certificate I get back to where I started with the issue.
Another thing I saw is that the IP of my system is out of range within the trusted_proxies list when installed with certificates. I tried editing it manually with nano (same as @Bender did) but after restarting the app from the GUI my change within that array is removed by the system. I might be completely wrong with this lead and I am not sure if this has something to do with the whole issue but this is where I am now.
I finally got it to work, but I’m sad to say that had to go back to Truenas 24.04 (dragonfish)
Not sure about where the problem comes from. It works smooth but whenever I update the system to Electric Eel, it automatically shows the untrusted issue.
As a conclusion… I’ll have to stay in Dragonfish