The credentials it is asking for are set for the desktop app install you are using.
If you go to your users Home directory and view the hidden files there will be an n8n folder, In there you will find a file called
n8n-desktop.env this will contain the credentials you are looking for.
On Windows you can just pop
%HOMEPATH%\.n8n into the file explorer to get there if needed as well.
For Mac (and Linux when released) you can just pop open a terminal window and run