Assuming you will be doing email distribution through contact list, you can add an embedded data in the contact list and assign unique value for each contact. Then in survey, you can read this value.

Thanks a ton for your reply. I would try that. However, if you can give a me an example how to do that, it will be of great help. Suppose I have a list of five subjects in my contact list and their emails are:
abc@gmail.com
def@gmail.com
ghi@gmail.com
jkl@gmail.com, and
xyz@gmail.com
I want to assign IDs 10011001, 10011002, 10011003, 10011004, and 10011005, respectively. How would I do that. I am assuming that the survey option I will have to enable is "By invitation only".
Thanks!
Regards,
Ashwini

I am assuming that the survey option I will have to enable is "By invitation only" - Yes, this comes under best practises when you are distributing via email, but will not solve our requirement.
Step 1: Check step 8 of this and add embedded data "ID" to your contact list.
Step 2:https://www.qualtrics.com/support/survey-platform/survey-module/survey-flow/standard-elements/embedded-data/#SettingValuesFromAContactList

Thanks! In a cursory look, it seems, if I want only an ID associated, the External Data Reference column may be sufficient. However, other demographic info (which I want), the Embedded data is the way to go. I will go through in detail and update.
Regards,
Ashwini