I just got cookies going after a fair bit of trial and error.
I added two entries, one a path for the cookies folder, and the other is an environment variable to specify the YTDL_OPTIONS.
Path:
Host Path: /mnt/user/appdata/MeTube/cookies
Environment Variable:
Key: YTDL_OPTIONS
Value: {"cookiefile":"/cookies/cookies.txt"}
Exported my cookie file from my browser, put it into the correct folder on my server, restarted MeTube and it works.