It's TN, then your birthday (day, month, last 2 digits of year), then M or F, depending on whether you are male or female.
I was born on 14 Aug, 1965, so my temp number was TN140865F.
I didn't need to show my NIN till after I was hired, though, so I don't know if you'll need it for an interview. I suppose it doesn't hurt to have it with you.