So, here is some quick steps from which you can add an extra username for your charter wbemail account"

• Open your browser and login to your charter webmail account.
• Now go the my account option on the top right of the screen
• Click on the add user and accept the T&C Provided there
• Now you can create your unique username 
• It will ask for a primary username, select whatever username you like for primary purpose
• Now click on save & you've successfully create additional username of your charter.net account!

If you're still struggling in signing up or login to your charter webmail account, then you can read this article: https://emailscrunch.com/charter-spectrum-email-login/