Changing text block under log-in box

Running iMac 27", mid-2011 series, High Sierra, 2.7 GHz i5

Under my user log-in box when I startup my iMac, I previously included a sentence about contact information. But now I can't remember how I did that and I want to delete it. It doesn't appear when coming out of sleep mode--just when I log on after start-up. Any help would be appreciated.