The option is already there for you :)
If you look in your Online Chess settings page or your live chess settings page, check the box marked 'show algebraic notations' then save changes before leaving that page.
Resolved: Letters and numbers on side of board
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On some chess boards IRL I've noticed the squares are marked with the letters a-h and 1-8. Can we have an option for this in chess.com? Thank you!