var/savefile/F = new("Admin")
Admin = list(Admins = "Xyren" = 4)

Problem description:
When I use this it creates an empty savefile called Admin. However I want it to write Admins = list("Xyren" = 4)
On one line within the file. What am I doing wrong?
Your initialization of the Admin list is invalid (which should be obvious as it doesn't compile). It should just be list("Xyren"=4).

More importantly, however: you are not actually DOING anything with the savefile. In order to store values in it, you need to use the << operator (see: Reference)

Additionally, savefiles are not plaintext. You will not get a savefile containing the string "Admins = list("Xyren" = 4)". If you want plain text, you can output to a textfile using text2file() and file2text(), but then you have to handle serialization and such on your own (you don't want to do this). You can read a savefile in a plaintext format using ExportText(), but that's not actually particularly useful except for simply examining the contents of a savefile.
You solved your own problem within your Problem Description. Does the bolded text look like what you put into your snippet?