Assigning F keys a value

Back in the day of DOS, Win 3.0 and 3.1, I was able to assign a vaue such as a password to an F key. After searching the web extensively and contacting Micrsoft I am unable to get an answer.
Does anyone know how this can be done? There a F keys whose function I never use and I fail to understand why the function I don’t use can’t be eliminated to leave that key available for my own assignment. Thanks for your time and attention.
Jack