Create Contact Custom Field
To add a Custom Contact Field, from the Dashboard click on Accounts, then scroll to the right side. Click on Custom Contact Fields and then click "Add New".
Provide a Name, Key, and Description for the Custom Field.
To check your newly created Custom Field, you can go to your contact's tab, choose one from the contact list, scroll down and look for the Custom Field tab. From there you can add, or edit any information that you want for your contact.
The custom fields can be useful if you plan to add identifiers to your existing contact or contact list. It can also help you with reporting and tracking.